According to characteristics and demand of fine tracking spot detection in space laser communication system, a kind of real-time spot detection instrument was brought forward by adjusting gain automatically , which can overcome saturation photic intensity effect. Function of calculation center of spot, smoothing filter et al. was implemented by using DSP for spot change. By experiments, miss distance can be exported in real-time, and gain can be adjusted automatically with the change of input powers, which can overcome the failure brought about by saturation photic intensity. Shift characteristic curve of four-quadrant detector is measured, which has been proved that 10.1 μrad of angle resolution has been obtained. Furthermore, the ability of subdivision can be double by digital filtering. The parameters of filter order, orthogonal compensation angle etc. can be set from external configuration.