A method to determine directional angle of light source and installation location was proposed. Using transmission characteristics of underwater rays of light and considering transmission distance, volume scattering function, scattering angle and the small volume unit, the illuminating effect of various directional angle of the light source were analyzed. The selection principle of directional angle of the light source was proposed. It was that when meeting the requirements of the monitoring area, dirctional angle of the light source should be chosen to make the scattering angle maximum. When the view field angle was 45°, directional angle of the light source should be 67.5°. According to the boundary of the monitoring area and directional angle of the light source, the installation location of the light source was determined. The real results show that the system can meet the requirements of monitoring and has good effect.