Abstract: Aim To study the synthetic fuzzy algorithm applied in fault diagnosis expert system. Methods According to many cases in the normal inference and conjecture inference, the method combining the back symbol inference and forward digital inference was adopted. The input interface of evidence was designed using the fuzzy theory. Results A synthetic fuzzy inference algorithm for fault diagnosis expert system was advanced, and the FMS fault diagnosis expert system was developed. Conclusion By the analysis of theory and application, the fault diagnosis algorithm has the characteristic of universality, completeness and high efficiency in the fault diagnosis field.