An improved method of Performance Evaluation of Weapon Systems (PEWS) is presented based on fuzzy arithmetic operations with revising the defects of the original method. PEWS is regarded as a fuzzy MCDM problem. Five fuzzy ratio scales are defined to represent the elements of fuzzy judgment matrix and weight vector according to the data in the "Goal - Criteria - System" structure, and then PEWS is transferred into fuzzy ranking scores. The comparison of the three results obtained respectively by this new method, the original method and the relative grade method shows that the new method is credible and effective.