The earthquake liquefaction threshold is a very important scientific problem. It is generally believed that the earthquake of magnitude 5 will not result in large- scale liquefaction. On December 14, 2009, a moderate 5. 1 magnitude earthquake struck Hami City in western China’s Xinjiang Province, with a focal depth of only 4 kilometers (Figure 1). This paper is to discuss the mechanism of extensive liquefaction caused by Hami 5. 1 earthquake. Methods:The authors made a detailed field investigation in Hami, Xinjiang, and obtained a lot of valuable geological information, especially sand dikes and obtained a lot of valuable salt solution activation data through the laboratory fluidization experiment and salt solution activation experiment, which provided valuable data for further study of liquefaction activation of saline solution. Results: The liquefaction boundary can be 80 kilometers away from the epicenter, and may even reach 120 km, which is equivalent to the epicenter distance of a 7. 0~8. 0 magnitude earthquake. It is undeniable that the formation of Hami sand dikes, especially the unusually significant liquefaction and fluidization. Seismic sand dikes grid plane from tens of centimeters to more than 2 meters; The longitudinal section is wedge- shaped and oblique (average 75. 10°), and the sorting is good. The dip Angle of sand dikes ranged from 52° to 90°, with an average of 75. 4° (250 sand dikes were taken as examples), and more than 96% of sand dikes had steep dip Angle (inclination >60°). The Hami sand dikes are convex in both plane and section, but are flat and even grooved in some places due to flood erosion. The protrusion height of hami sand dikes is generally 2~3 cm ~ 5~6 cm, and the maximum is 10 cm. The experimental results show that under the condition of salt water concentration of 23% and 9%~17%, the shear stress inside sand dikes can be greatly reduced by salt solution, which is 52. 63%~85. 20% and 12. 51%~21. 58%, respectively, which is conducive to the formation of sand dikes by seismic liquefaction and fluidization. An unusual assemblage of sedimentary facies and basement (sandwich- like assemblage) is thought to facilitate large- scale liquefaction. In addition, solid salt dissolves to form a thinner fluid layer that can form an overpressured fluid on its own without seismic liquefaction. Conclutions: ① A large number of sand dikes are developed in the study area, which are continuously distributed, medium—small scale, polygon on the plane and wedge on the section; most of it leans. It is believed to have been a moderate earthquake of M5. 1 on December 14, 2009. ② Due to the shallow basement, thin cap layer, and intense salinization, Hami, under the very limited shock impact of such a moderate intensity earthquake just entering the liquefaction threshold (M5. 1), formed a wide distribution of seismic liquefaction sands extending at least 80 km outward from the epicenter, and possibly up to 120 km. ③ The good characteristics of dike sorting are definitely due to the liquefaction and fluidization sorting of SSGL and SGSGL, rather than a pure parent sand unit. Through fluidization experiment and calculation, fluidization distribution and fluidization boundary diagram of fluidization sand with low particle size are obtained. ④ Experimental studies on the minimum fluidization velocity (Umf) of particles <0. 125~40 M are reported, and two empirical formulas are proposed: Umf=6. 612×D0. 6277 and Umf=7. 7443×D0. 6293. The maximum fluid excess pore pressure and seismic shock pressure are calculated to be about 513. 448~637. 29 kg/m2. They are quite different from the classical formulas. The maximum fluid overpressure and seismic impact pressure (about 513. 448~637. 29 kg/m2) were calculated from the overpressure values of 40 M gravel and the elevation of sand dikes. ⑤ The study shows that the sand forming sand dikes mainly comes from the fluidization sorting of the sand and gravel layer below the bottom of the dike. ⑥ The reason why Hami can form long distance sand veins under M5. 1 earthquake is mainly due to the following six advantages: ① The salt solution of sand mud sand is easy to liquefy and fluidize. Concentrated brine and semi- dissolved salt can reduce the shear capacity of particles by 25%~75% on average, and the seismic liquefaction threshold is reduced to 0. 15~0. 05 g (with 0. 2 g as the general threshold). At the same time, with the increase of density, the minimum fluidization velocity (Umf) of fresh water can be reduced by 12. 51%~21. 58%, which is conducive to fluidization. ② Widely distributed SSGL and SGSGL; ③ The source is very shallow (only 4km deep); ④ very shallow basement (depth of about 0~3 m); ⑤ The special sandwich structure of the salt—debris- mixed- cover +SSGL+SGSGL+ extremely shallow bedrock basement. Based on the excess pore pressure value of 40M gravel and the height of sand veins above the ground, the maximum fluid excess pore pressure and seismic shock pressure are calculated to be about 513. 448~637. 290 kg/m2.