Abstract: The authors inverted density anomalies on the different layer in the depth range of 10~100 km in Sanjiang Region,using gravity field that is treated by separation and adjustment.The results of combining other geophysical data to analyse the density data show that:1) The material in Sanjiang Region possibly comes from the extension of the Qinghai-Tibet Plateau;2) Low velocity layer exits in the crust of Sanjian Region;3) The earthquake easily occurred at the inflection points of density and gravity anomalies in lithosphere,especially at the complex inflection.