In order to solve the technical problem of the sensitivity of the flame trigger drops sharply under strong background light，master-auxiliary detector flame trigger technology is put forward in this paper. At first，several factors which influence the sensitivity of flame detection are analyzed. The definition of sensitivity which is suitable for flame trigger in engineering practice is described and the mathematical expression of detection sensitivity of flame trigger is established. According to the influence of flame trigger sensitivity based on background light，master-auxiliary detector circuit is devised. The masterdetector is used to detect the target fire light signal，the auxiliarydetector device is used to detect the brightness of the background light，it also provides the basis for the master detector to realize the gain of signal processing circuit and the adjustment of lens incoming light. The master-auxiliary flame trigger is verified by experiments. The result shows that the master-auxiliary flame trigger can effectively detect the radiation light of firecrackers beyond 85m，anditssensitivityfullymeetstherequirementsofthelong-distancetestoffirelighttriggerinweapontest.