Abstract: In order to improve the guidance method for torpedoes,based on laser Doppler effect,a theoretical model is established for detecting the ship wake with laser.A differential optical system is designed to produce Doppler shift frequency in scattering signal on sensor,which can improve the signal-to-noise ratio efficiently.The simulation results indicate that the Doppler shift signal can be detected by lock-in amplifier technique,and the noise will be limited.The technique has a broad applicative perspective in under-water weaponry.