Based on possibility theory of fuzzy mathematics, we put forward a new conception-fault possibility and deduce it's fuzzy logic expression. According to the fault possibility distribution in a serial model system, we present a new method for system fuzzy synthetic judgement. At last, by using a serial model system for example, we discuss how to apply the result which we've got ten in this paper to acquire the relative data for fault diagnosis and fault located steps in the receiver part of a radar system.