Abstract: To develop a kind of valid formalization method, and to use incomplete knowledge to obtain important results, this paper provides a method called "Reliability-Enhanced Fuzzy Qualitative Simulation", which combines qualitative reasoning, fuzzy sets and theory of probability together. The method uses qualitative reasoning to build models ,fuzzy sets to describe the common sense knowledge, and theory of probability to solve the uncertainties in simulation ,and gives the most possible qualitative behavior under the user's current description.