Under the complicated scene,the weak infrared target detection is one of the key technical problems in the infrared target recognition.This paper uses the probability theory to analysis the background factor′s role.Then the membership theory on the fuzzy sets and the fusion theory of the multiple classifiers are introduced to design the background factor.And the complicated background is adaptive weakened because of the background factor.At last the detection probability under various background tends to the uniformly distribution,and the constant false alarm rate is achieved.