Abstract: The perfection of comprehensive quality assessment system is very significant for effective implementation of comprehensive qualities assessment work and for the healthy development of college students. The key of the student comprehensive quality assessment system is the conversion from credit to comprehensive quality assessment. By analyzing the present formula of credit，this paper finds out the main defect is that the conversion formula doesn’t conform to the statistic theory and points out the conversion formula of students credit should be based on the numerical analysis of the characteristic parameters of specific students group. Testing the credit distribution data of the students in a certain major and in a certain academic year in Southwest Petroleum University，this paper builds up a model based on statistics principle，constant variation law and fixed point theorem，and by dividing characteristic parameters group of credit assessment by means of derivatives，making numerical integration to each phase，and determining variation items through normalization，we get the new conversion model. The result shows that the new model can better reflect the importance of credit in the comprehensive quality assessment.

Key words: credit, numerical analysis, constant variation law, fixed point theorem, comprehensive quality assessment of university students