According to the characteristics of fault data of CNC(Computer Numerical Control) machine tools and from the point of view of life cycle, an evaluation method based on DEMATEL(Decision Making Trial and Evaluation Laboratory) weight-TOPSIS(Technique for Order Preference by Similarity to an Ideal Solution) method is proposed. In this method, the relationship between the RPN(Risk Priority Number) calculated value of the design FMECA(Failure Mode Effects and Criticality Analysis) and the RPN calculated value of the process FMECA is used as the input data of DEMATEL to obtain the weight required by TOPSIS, and then uses the improved TOPSIS distance formula to sort the failure modes. This method is used to analyze the fault of servo drive system. Compared with the traditional RPN method, it overcomes the mathematical defects of the traditional RPN method and determines the weak parts of the servo drive system. The results of calculation and analysis show that the evaluation results of this method accord with the actual situation of servo drive system, and have certain practicability and feasibility.