Fouling coefficient is an important measure parameter of condenser performance, but it is affected by multi-factors, therefore judging the condenser water side of tube walls fouling based on coefficient would have a significant error. So this paper proposed the concept of degree of cleanliness, which allows us to analyze the influence of the air accumulated on the steam side、the fouling on condenser water side of tube walls、and the tube bundle coefficient of condenser on the overall heat transfer coefficient of the condenser respectively. It can diagnose the condenser fouling more accurately and provide a basis for scheduling reasonable cleaning. Experimental results show the method can be more reliable than the T-S fuzzy model, thermal resistance method, RBF neural network model when the condenser pipe blockage or a larger amount of air leakage into the condenser or condenser operating mode parameters change rapidly.