Objectives: Based on the crustal density images obtained in Qinghai—Xizang (Tibet) Plateau that we presented by previous papers, a study of causes and sources of lowdensity belts in the studied area is analyzed fin this paper. Methods: The basic theory about thermal stress in continuum physics is applied for the analysis and links to data of lithospheric investigations, aiming at formation mechanics of lowercrustal channel flow. Corresponding functions linking the thermal strain, temperature and density variation in the Lower crust provide the possibility of estimating the Lower crustal thermal strain maps, providing important evidences for study of the channel flows and mass movement. Results: Applying the methods produce the results that the maximum thermal strain of this channel flow caused by heat expansion equals to 15.5mm 3/a. And the resulting map shows that positive thermal strain indicates expansion areas of the Lower crust in the studied area, whilst negative thermal strain indicates cold contraction areas. The lowercrustal channel flows originate from two sources that characterized by areas of the highest thermal strain, then flow into areas of the lower thermal strain. The areas of the highest thermal strain are located at two places, the first is by the eastern side of the Karakoran fault between the Bangong—Nujiang and the Yaluzangbo sutures. The second flow source is located by the northern side of the Yaluzangbo suture between Lhaze—Nyingchi. The channel flows originates from the first source goes to NE and NW, i.e., to Aljin area and Karakoran area respectively. The channel flows originates from the second source goes to east, reaching to Litang—Yajiang area and North Yunnan, then diversely flows to north and south. Conclusions: The study supports the hypothesis of the Lower crust channel flows and diapiric intrusions of the flow from the Lower crust into the middle crust like dental cream, which have been resulting in break of gravitational equilibrium and uplifting of mountains in Qinghai—Xizang (Tibet) Plateau.