Firstly, the index system influencing cleaning quality was established based on AHP theory and the weight were calculated. According to the weight value, the key factors which affected cleaning process were determined. Secondly, the model calculating the weak degree of index factors was established by introducing a basic concept (eligible degree) from extendable optimal degree. According to the weak degrees, using the method for determining the number of principal components in principal components analysis method, the weak factors were determined in key factors. Finally, taking the cleaning process of cooling system of the car diesel engine in some repair factory for an example was analyzed, the result showed that the method to determine weaknesses was effective.