Over the past decade, unconventional oil and gas has become an increasingly important resource in global energy structure. Low-permeable and tight oil has become a hotspot issue in exploration, which has considerable prospects. The research for start-up pressure of the low-permeable hydrocarbon accumulation has been a difficulty, but it is still in the start stage. It is directly related to the theoretical research and exploration of low permeable and even tight hydrocarbon accumulation. The changes of petrophysical property under overpressure condition and the relationship between the throat sizes and porosity were discussed in combination with core analysis of the Upper Triassic Yanchang Formation in Ordos Basin. The results show: there was a good power relationship between effective pressure and porosity and permeability of sandstones with super-low permeability; coefficient of fitting function has a good correlation with the surface reservoir physical properties; the throat sizes has a good correlation with the reservoir physical properties as well, which has showed an exponential relationship with property, and a power relationship with permeability. The calculation method of start-up pressure based on permeability is more reliable to reservoirs which are characterized by “compacting before charging”, while it needs further research for the calculation method of start-up pressure for reservoirs which are characterized by "charging before compacting" or "charging while compacting". Finally,analysis about chang7 reservoir of Yanchang formation in Huchi—Heshui area shows that under the reservoir condition the permeability is (0.00473~0.52832)×10-3μm2 with an average of 0.01380×10-3μm2; the property is 1.458%~10.851% with an average of 7.224%，and the average throat radius is 0.0033~0.3769μm. This area is characterized by "compacting before charging", and the actual start-up pressure is 0.0788~13.0022MPa with an average of 2.1025MPa. The Chang7 source rock is of a maximum burial depth of 3000m, the charging pressure is 6.06MPa，so the throat radius and permeability cutoffs under reservoir condition is 8.576×10-3μm and 0.00486×10-3μm2。