An underwater image detection theory of inhomogeneous illumination field is proposed to reduce the backward of scattered background noise. The distribution function of inhomogeneous illumination field is derived. It is related to receive aperture, distance between target and receiver as well as seawater volume attenuation coefficient. An underwater image detection system with distributive light of central power is developed. The tank contrast experiment is proposed, which verifies the inhomogeneous illumination field′s distribution characteristics and underwater probing characteristics. It can clearly distinguish 1 mm details at 0.6 times visibility and identify targets outlined at one times visibility, as well as detect the target at 1.5 times visibility.The underwater system based on inhomogeneous illumination field theory is effective to overcome the influence of backward scattered background noise. These results suggest that this system has a far detection distance, wide visual angle, full view depth and good image definition.