A theoretic model was built about the distribution of optical backscattering energy on the two dimensional photosensitive surface. Then, applied to typical underwater laser imaging system, a method was proposed to calculate the maximum detecting distance according to the apparent contrast. The theoretic calculations were programmed, and the variances of the apparent contrast were discussed when changing the relative position of some components. The relationship curve between the apparent contrast and the target distance as well as the curve between the maximum detecting distance and the attenuation coefficient were obtained, so that the maximum detecting distances could be estimated in different kinds of water quality, which are consistent with the actual experimental results.