Abstract: Since the final stage of Cretaceous, the Xihu Sag has undergone paar stage, depression stage and regional subsidence stage, developing several extensive surfaces of unconformity (seismic reflection sequence:) T₂⁰,T₂⁴,T₃⁰, etc. This paper explains the basic principles and calculating methods of obtaining erosion amount using sonic velocity and vitrinite reflectance data of the Xihu Sag. It also analyses the influence of strata erosion on petrloeum accumulation, probing into the applicability of those methods.