The Cenozoic sedimentary layer is widely distributed in Xiaochang Coal Mine in the study area, with a thickness range of 153 ~ 190 m, which is in an unconformity contact relationship with the underlying bedrock. Affected by the drastic fluctuation of strata, the velocity difference is significant in the process of time-depth conversion. In order to improve the interpretation accuracy of coal seam floor and control the structural swing error, this paper compared and analyzed the imaging effect difference between pre-stack depth migration and pre-stack time migration in view of the complex geological conditions of large Cenozoic sedimentary thickness and violent fluctuation of underlying bedrock strata. Through theoretical derivation, comparative analysis and practical application cases in Xiaochang area, the advantages of pre-stack depth migration in velocity modeling accuracy, structural recovery ability and time-depth conversion error avoidance were demonstrated, which provided technical reference for seismic data processing under similar geological conditions.