Abstract: To study the effect of rotational speed on the performance of Roots pumps, the internal flow in a Roots pump under different rotation speed conditions was numerically simulated by using the Reynolds time-average method coupled with SST k-ω turbulence model, and the influence of rotational speed on the internal flow and pressure pulsation of Roots pumps was analyzed. The results show that with the increase of speed, the outlet pressure and velocity of the Roots pump increase gradually, the vortex strength of the flow channel at the inlet is basically unchanged, the vortex strength in the rotor closed cavity increases, and the vortex intensity at the outlet decreases gradually; and the rotor angle has a great influence on the flow field structure in the pump due to the change of the rotor cavity volume. The exhaust volume of the Roots pump will fluctuate four times in one rotation cycle, which is consistent with the number of rotor blades, and the exhaust volume increases linearly with the increase of rotation speed. There are four pressure pulsations at the inlet and outlet of the Roots pump in one rotation cycle, and the main frequency of the inlet and outlet pressure pulsation is the blade frequency, and the pressure pulsation amplitude at the main frequency will gradually increase with the increase of speed.