As the oil pressure of tank engine’s common-rail system is as high as 180MPa, it could bring a grate challenge to the oil con-rail’s seal quality. Especially in the condition of high oil pressure system, pulse load, metal creep deformation as well as machining error could low the reliability of con-rail’s. In tradition, shrinking the part’s tolerance is the main measure to ensure seal reliability .The measure eventually cause low efficiency, unnecessary waste, and high product cost in mechanical process. In this analysis, ABAQUS is exploited to discuss the influence that caused by various machining error on seal quality .Finally , the least taper angle error (critical point) lead to leakage is found. Besides the efficiency would be improved, the process cost also could be cut down a lot by using this data to constitute optimal technical files.