MATLAB is used more and more widely in digital image processing. First, the original test data obtained from underwater laser line scanning system need to be processed. The mathematical morphology can be used to deal with the underwater laser line scan results. The background light noise can be eliminated effectively. The targets can be extracted from the histogram equalization. The experiment results show that the resolution and SNR are improved, and the quantity of underwater laser line scan image is improved. At the same time, the detection range of underwater laser imaging is increased.