Abstract: Based on the exploration practices in north and middle regions of Sichuan Prouiace, this paper tries to use the theory of the compaction of muds tones and the migration of fluids and probe into the relation between the fluid pressure in mudstones and the accumulation of oil and gas. The result of the isodepth calculation based on the curve of seismic time difference versus burial depth indicates that the mudstone source beds of Qianfoya Formation and the Daanzhai Member of Ziliujing Formation in northern Sichuan have abnormal high pressures. Primarily,it is believed that it is resulted from the inhomogeneous compaction of mudstones during the time from the beginning of Qianfoya Period to the end of the Late Shaximiao Period; and also from the integrated effects of various factors since then.