This paper analyzes the conditions for the formation of sealing of a fault，According to the seismic，geological and log data and by applying the technique of fuzzy comprehen— sive jugement，the fuzzy comprehensive judgement matrix and mathematic model for the sealing of a fault have been established．This model is applied tO the study of the sealing of main faults in the Qiaokou Oil Field，and the barrier of a fault to hydrocarbon accumulation in this area．A set of methods suitable for evaluating the Sealing of a fault in different stages of exploration and development have been set up．