Abstract: To investigate the influence of grid size and water size on the accuracy of three-dimentional (3D) numerical simulation of underwater explosion, and to give the parameter settings that can balance the accuracy and efficiency of 3D numerical simulation of underwater explosion, this paper is based on the LS-DYNA 3D free field model of underwater explosion, using the peak of underwater explosion shock wave, bubble pulsation characteristics and water motion characteristics as indicators, in 50-300 kg explosion equivalent, and analyzed the influence of grid size and water size on the accuracy of numerical simulation respectively. Results show that the grid size is not more than 1 times the radius of the charge, the distance from the center of the explosive to the boundary of the water is not less than 4 times the maximum expansion radius of the bubble, and the software can be more accurate simulation of underwater explosion shock wave and the first bubble pulsation of the whole process. Considering the calculation accuracy and efficiency, it is recommended that the grid size should not be larger than 1 times the radius of the charge and the distance from the center of the charge to the water boundary should not be less than 4 times the maximum expansion radius of the bubble when the numerical simulation of underwater explosion is carried out.