Igneous rock of Permian, which velocity and thickness change dramatically, is widely distributed in Tahe area. The variation directly result in rapidly shake in structural characteristics of underlying strata, and causes inheritance pseudo-structure. It brings difficulties to the identification and interpretation of the Ordovician fractures and caves. This paper is based on depth migration algorithm, and focusing on velocity modeling for igneous rocks imaging in the study area. A high-precision velocity model is constructed by variable-scale grid tomography inversion and multi-information constrained target inversion, then the velocity is used for migration. Compared with traditional grid tomography method, this way can depict igneous rock velocity more accurately, effectively eliminate inherited pseudo-structures and pseudo-faults in the underlying strata, moreover, recover the true underground structural form, and enhance the precision of small fracture and cave imaging in the interior of Ordovician.