The analysis of the terminal divert ability of SM-3 is put forward based on the creditability theory. The expression of the maximal zero effort miss which could be corrected by SM-3 during the terminal course was deduced based on relative motion dynamics. Creditability theory was used for modeling the fuzzy parameters in the expression, and fuzzy simulation was adopted to obtain the possibility measured pessimism value and optimism value, creditability measured mean value and normalized variance, and its distribution functions of possibility, necessity and creditability. The research could supply a theoretical support for the motor design of midcourse maneuvering penetration of a ballistic missile, and supply an effective way for get reasonable unknown information from limited known information.