Comparative to the vitrinite reflectance (Ro) of the Cenozoic, the value of Ro from the Upper Paleozoic displays distinct anomaly. Based on the analysis and comparison of a number of Ro data, this study established maturity equations (Ro H) of the Jiyang depression of both the Cenozoic and the Upper Paleozoic, through which the eroded thickness of the discontinuity surface between the Mesozoic and the Paleozoic in the Jiyang depression was restored. The result revealed that the eroded thickness of the discontinuity surface in the Dongying Sag, Zhanhua Sag and Chezhen Sag are 3.1~4.1 km, 2.0~4.1 km and 3.1~4.1 km, respectively, averaging 3.5 km. This fully suggests that the North China Plate was a unified largescale sedimentary basin before the IndoChinese movement, and part of latePaleozoic coalbearing strata started to produce hydrocarbon by means of thermolysis.