Abstract: Dependence of image contrast on atmospheric back scattering by continuous coaxial illumination is analyzed. As a result, several equations on imaging contrast are given and relation between image contrast and weather is studied. Scattering flux is calculated by Mie scattering theory on condition that relative scale and refractive index of the particles are known. Dependence of image contrast on weather is presented in this paper. Theoretical results accord with experiments and indicate it is impossible to obtain distinct image of target through long path of atmosphere under coaxial continuous illumination when visibility is not very well. So range-gated method is necessary in active imaging system.