In order to obtain the precise three-dimensional (3-D) points of underwater objects, we propose a corresponding pixel matching algorithm based on coplanar constraint for underwater laser scanning systems. It uses three coplanar constraints of left camera, right camera, and laser plane to achieve high-precision and high-speed pixel matching according to the characteristics of underwater laser scanning system. Results show that the measurement error is <3  mm for the bat placed 3 m away and compared with a method using epipolar constraints. The proposed method can greatly reduce the measurement error by 89.64%. In addition, the average processing time of each image is 5 ms, and the generated 3-D points are uniform and dense, which means the system can realize real-time processing and good robustness with high precision.