In the traditional photoelectric tracking system, the fixed scale filter is usually used for target detection. The target features especially the target size is changing all the time due to the distance change between the target and tracking system. It results in the filter not suitable for the target size changing, and it is difficult for the system to obtain the best extraction results. For this question, a small target detection method based on adaptive scale is proposed. Based on the Laplacian scale-space theory, the normalized Laplacian scale-space images are studied, the target signal is enhanced and the noise is suppressed at the same time. By finding the best parameter of target both in the scale and space, the small targets with different scale could be detected effectively. The simulation test and the experimental results show that the method proposed improved the detection capability of the system for small target with different size remarkably.