In order to break the limitation of the high speed on-off valve stroke on the switching frequency, a kind of rotary high speed on-off valve was proposed. Combining theoretical calculation and CFD simulation, the influence of the change of spool structure parameters on the overflow area, flow coefficient, jet angle and hydraulic moment of the valve port under different spool rotation angles was studied, and change laws of the hydraulic moment were obtained. The results show that the hydraulic moment is proportional to the two squares of the pressure difference and flow rate of the valve port; when the pressure difference is timed, the hydraulic moment is proportional to the overflow area of the valve port and the cosine value of the jet angle, and with the increasing of rotation angle of the valve core, the hydraulic moment first increases and then decreases; when the flow rate is fixed, the hydraulic moment is inversely proportional to the overflow area of the valve port, and with the increasing of rotation angle of the valve spool, the hydraulic moment gradually decreases to zero. It is proposed to change the jet angle by adjusting the height of the spool groove to achieve the purpose of compensating the hydraulic moment.