Abstract: Bei’er depression experienced a few tectonic movements from extensional to contractional and shaped 5 types of unconformities characterized by truncation unconformity:parallel unconformity、truncation unconformity、lapout unconformity、fold unconformity and faulted-fold unconformity. Using vitrinite reflectance,the author calculated eroded strata thickness of 29 wells, whose results are different due to varieties of the control factors. Finally,this paper obtained that eroded stratum thickness of T₃ reflector of the well Bei6 that lied in south-eastern margin in Bei’er depression was 260 meters.