Abstract: By considering the difference of all kinds of optical paths, the echo signal from plane target in underwater detection is analyzed. The power equation of the echo signal is deduced. The factors to influence the signal power, such as the size of target, the parameters of water and the angles of incidence etc., are simulated and analyzed. The result indicates that the received power will reduce as the transmission distance and the parameters of water increase; but it will increase as the size of target increases.