In anti-hunting early warning system, laser warning equipment was mainly used to identify the orientation of enemy laser signal. Water′s laser attenuation nature would reduce performances of alarm, its light absorption nature would lower SNR of the receiver and its scattering character would reduce the orientation precision. Firstly, transmission equation theory was used to establish underwater transmission model of laser and then Fourier transform and plane wave expansion method were used to solve water transport equation. Lastly, light reciprocity principle and transmission equation were utilized to obtain the two-dimensional spatial intensity distribution model of the probe in the plane. Simulation had been used to calculate the response of detector to direct light and scattered light at different seawater environment in clean water, coastal water and turbid water. Results of the simulation show that the proposed method in this paper could be used to assess receptivity performance of the detector in underwater alarm system.