Detecting angle between target and optical axis in Q-APD is an effective way of fast ATP. It is the key point in this method to acquire the ratio of some parameters. In this paper, a novel method of achieving division of analog electric signal using integral chopper is proposed. This can avoid the problem of high speed acquisition faced in the digital processing solution, that is, MHz level sampling frequency should be used in microsecond level signal. After the target is detected by using Q-APD, the four-channel signals generated by target are amplified by the emitter follower. Then using sample hold and sum difference calculation, background signal and offset signals in azimuth and elevation are generated. Because of using the integral chopper, integration time of offset signal to capacitance is controlled by the background signal, and the output value is just the ratio of the background signal and the offset signal. This value which has is independent of intensity of incident light, represents the angle between the target and optical axis. The optical axis can be driven by servo system based on the angle, therefore realizing the fast ATP.