IR small target detection is always one of the topic difficulties, and it is easy to be covered by the background due to many factors. The IR small target detection is very difficult for the complicated background and stealthy technology application. With scale-space theory, the characteristics of IR images of the IR small target in the energy field are analyzed. Multi-step energy difference is used to determine the center point and the size of the target. Based on the characteristics of noise and target point, the isolated noise points in the results are rejected to inspect the small target finally. Experimental results show that the algorithm has simple calculation, short running time and can inspect the target effectively.