Abstract: According to lateral sealing mechanism of fault, this study establishes a quantitative evaluation method for ancient lateral sealing of fault in the key stage of hydrocarbon accumulation with a calculation method for present displacement pressure of fault rocks that considers compaction diagenetic pressure and time of fault rocks, and with a combination of techniques for recovery of ancient burial depth and evaluation of fault activity. The method is applied to a quantitative evaluation of ancient lateral sealing of fault B₂₉ at the stage of hydrocarbon accumulation in Beier sag, Hailaer Basin. Results show that in fault B₂₉, reservoirs (except reservoir No.8)are presently sealed,and reservoirs (except reservoirs No.8)were sealed laterally in the stage of hydrocarbon accumulation (end of deposition of Yimin Formation). The evaluation result is consistent with drilling result of Well Bei-302 in the study area. Thus, the proposed method is applicable for evaluating ancient lateral sealing of fault.