Abstract: In complex geological structure area，high precise underground structures can′t be gained only by prestack time migration.Prestack depth migration is an effective way in gaining accurate image of complex structure.To gain accurate prestack depth migration velocity is a key technology of prestack depth migration.Because there is close relation between velocity and depth，the velocity can′t be calculated directly，and it is only calculated gradually.The process of calculating the velocity includes three times migration velocity scanning and eight types of velocity calculating，which is a gradually convergent process.Whether the velocity is correct is judged with stability and validity.Some existing seismic data of Renqiu buried hills are processed for prestack depth migration with self-developed BGSS System.Prestack depth migration image can display a clear internal structure of the buried hills.Compared with actual drilling information，the maximal relative depth error of prestack depth migration sections is 1.29%.

Key words: prestack depth migration, migration velocity, velocity scanning, velocity alternation, high resolution-imaging