Underwater range-gated imaging is an important method of underwater photoelectric detection. In order to obtain sufficient illuminating areas, an efficient model of the irradiance spatial distribution is derived for Gaussian laser illumination in the water based on small angle scattering theory. Irradiance distribution measurements of a 532 nm laser source with different divergence are presented. The errors of the model become large when the divergence of the laser increases and the illuminating range extends, but the resulting predictions of the analytical model are in good agreement with the experimental measurement over wide range of illuminating distance by limiting the field of illumination to the circle region with a diameter of 1 m. These predictions can give enough precision for underwater range-gated imaging systems.