The flow regime change of the deep spillway tunnel is determined by the downstream flow shape and opening mode of the service radial gate. Unreasonable arrangement may cause cavitation damage to the gate floor. In order to clarify the mechanism and conditions of cavitation damage based on the hydraulic model test and simple numerical simulation, the cavitation and its erosion characteristics of water flow under different shapes, water heads and opening angles were studied. The results show that due to the influence of the jet flow field distribution at the gate outlet, the gate floor pressure rapidly decreases with the increase of the total water head and the decrease of the gate opening in the downstream of the radial gate and the initial section of the downstream steep slope and the parabola, with the maximum negative pressure approaching or exceeding -14 kPa. With an extension of 5 m between the lock chamber and the parabola, the floor pressure can increase to a certain level, but the minimum cavitation number of the flow under the condition of high head and small opening is still relatively small, which is less than 0.2. If 0.2 is taken as the initial cavitation number of the parabola sections, meanwhile, the construction unevenness of the gate floor is controlled; the minimum opening of the radial gate can reach 1/4 when the total water head is 40 m. If the water head increases to 65 m, the radial gate can only operate with full opening or 3/4 opening, and the minimum opening can reach 1/2-3/4 in low altitude area or after the horizontal section is extended for 5 m.