Based on the principles of optical imaging and the method of triangular laser ranging, a method was proposed to measure the distance of an underwater target by three point laser sources and underwater camera. Theoretical models were built for the cases when the target was perpendicular to the laser beams, when there was a rotation angle and a pitching angle between the target and the laser beams. Mathematical relationships was established between underwater distance and geometric parameters of the laser spot pattern in the underwater image. Based on theoretical models, formulas for distance calibration were deduced. To verify the feasibility and accuracy of the proposed method, a prototype of the laser ranging system was implemented and tested in air and underwater. Experimental results show that the maximal measurement error is 35 cm and the mean value is less than 15 cm for a range within 8.4 m. The system has the potential to be used for accurate ranging at short distance underwater.