Abstract: The protection electrical potential distribution model of pipeline in cathodic protection and its reliability were discussed in this paper. Pipeline coating quality was evaliuated by in-site measured characteristics of cathodic protection. A new conception of coating interspace interspace coefficient was put forward which could express change of coating quality more clearly than "apparent electrical resistance". The method was applied to evaluate coating quality of pipeline from Haisan to Zhuangxilian station in Shengli oilfield which was verified to be correct and applied. It is important for external anti-corrosion of pipeline that coating e-valuation technology brought forward in the paper is combined with daily run of cathodic protection to apply.