Abstract: The software system of pre - stack depth migration of the American CoreLab Company is used to perform pre - stack depth migration of the seismic data acquired in the Southern .steep slop zone in Biyang depression based on the PC group parallel operation system. This paper systematically introduces the basic philosophy of velocity modeling in depth domain and the principles of pre - stack depth migration. In pre - stack depth migration profile, the main targets, especially the boundary large fault planes and the basement of the basin are clearly mapped and their locations are reliable; the interlayer contact relations - boundary between geological horizons and faults are clear; and the fault points of small faults are obvious, thus easy for interpretation.