Aimed at the features of gun-launched missile system fault and scarcities of fault diagnosis methods, the fuzzy-immunity mixed fault diagnosis method is proposed. The feasibility and innovation is theoretically analyzed, the overall project of fuzzy-immunity fault diagnosis is given, the math model of fault diagnosis is established; Taking guidance device as an example, the fault reason is quantitatively calculated. The diagnosis result indicates that the main reason of this circuit fault is protecting circuit; this method can resolve quantitatively diagnosis for gun-launched missile fault in the lack of prior knowledge.