The square soft-edge diaphragm with round angle is designed by Matlab, and is sent to a liquid crystal spatial light modulator by the computer. In order to obtain precompensation for the following laser system, local diaphragm transmission can be adjusted by feedback signals of surface-channel charge-coupled device (SCCD). This method can reduce the diffraction effect and realize no modulation, high stability, high homogeneity, and large scale laser beam. Several parameters of soft-edge diaphragms which affect the laser beam quality are studied systematically, and the optimized values are obtained. The method can avoid the serious modulation of hard edges and provide soft-edge diaphragms of different shapes in a fast and convenient way for the large scale laser beam system.