Abstract: In order to investigate the shape and formation mechanism of water plume, near free surface underwater explosions of 0.6g RDX spherical charge were simulated using the LS-DYNA software. Comparing shapes of water plumes and bubbles, parameters of bubble pulsation from the numerical and experimental results, the correctness of the numerical simulation method was verified. By observing the changes of velocity field between the bubble and water surface, the shapes of water plumes and bubbles, analyzing the corresponding relationship of the velocity field changes and the shapes of water plumes and bubbles, the shape evolution and formation mechanism of water plume were revealed. The study shows that the interaction between a bubble and free water surface is different from each other for different explosion depths, and it directly influent the shapes of water plumes. It also shows that the radial and vertical water column are forming when the bubble reaches the water surface at its expansion phase, and the water jet will generate no matter the bubble break up or not.