An algorithm for detecting the small infrared target was proposed. Analyzing the morphologic Top-hat operator,it is found that the small target can not be accurately detected only by relying on the Top-hat operator. Based on the Top-hat operator, a method of blocked adaptive threshold and target detection in small range are used to detect the small infrared target. And the Matlab simulation shows that the method can raise the success rate of detecting the small infrared target in a certain extent. By analyzing the result of two scenes, the method not only adapts in a single environment, but also has certain robustness in practical complex scene.