According to the requirement of equipment repair of certain type self parallel gun troops, designed a detection instrument for firing control system of a self parallel gun. Introduce the architecture, function modules and the working principle of the system. Then the design and realization of the hardware to the data collection modules and the experts diagnosis module are described. The paper presented the construction method of knowledge base which is base on fault tree and the reason control strategy which combined the forward reason with the backward reason. The instrument realized the quick localization of the fault for the fire control system of self parallel gun which meets the actual requirement to self parallel gun troops preferably.