The virtues and the ways of laser imaging for underwater target are described. A kind of underwater experimental system with micro-intensified CCD imaging and continuous blue-green laser-spot scanning is discussed, The physical model and computer formula of the system are analysed. The experimenu show that when it works under the contrast limitation model, its [1+(cos θ)/3]CRmax (ie. attenuation length-AL) value kept eontant. When water quality and target are certain, the value may become a basis for criteria of system suporiority.