Abstract: The paper divides crust into two layers: the elastic brittle layer and the viscoelastic ductile layer. Meanwhile, it supposes that the elastic brittle layer approximates to elasticity, and the viscoelastic ductile layer is viscoelastic. The paper calculates strain fields use GPS horizontal velocity, and deduces a strain relation expression between two points. The paper also evaluates interior temperature field of lithosphere by steady heat conductive equation. Under the condition that fracture intensity is equal to creeping intensity, brittle-ductile transition plane is obtained. And calculates varied depth effective viscosity of Tibetan plateau lithosphere. The results show that brittle-ductile transition plane generally locates in middle crust about 22~37km depth. The thicker the crust, the deeper the transition floor. Effective viscosity of middle crustal located in Tibetan plateau is between , and lowe crustal is between . It decreases to at Moho plane.