Based on the result of deep seismic sounding and gravitational data, after makingtopographic correction for gravitational data and Mesozoic-Cenozoic deposition corrections, the deep structure of Lanzhou-Tianshui region. (E103°-107°, N33.5°-37°) was studied by using Parker-Oldenburg's 3-D inversion method of gravitational field. By synthesizing and analysing the geophysical and geological data of the region, it is pointed out that the region is a transitional belt of Qinghai-Xizang block, moreover, we think that that Longzhong basin is a fault block's structure one.