The flow in the positive displacement blower is very complex. The existing two-dimensional numerical simulation cannot provide the detailed flow information, especially flow characteristics along the axial direction, which is unfavorable to improve the performance of positive displacement blower. To investigate the effects of spiral inlet and outlet on the aerodynamic performance of positive displacement blower, three-dimensional unsteady flow characteristics in a three-lobe positive displacement blower with and without the spiral inlet and outlet are simulated by solving Navier-Stokes equations coupled with RNG k-ɛ turbulent model. In the numerical simulation, the dynamic mesh technique and overset mesh updating method are used. The computational results are compared with the experimental measurements on the variation of flow rate with the outlet pressure to verify the validity of the numerical method presented. The results show that the mass flow rate with the change of pressure is slightly affected by the application of spiral inlet and outlet, but the internal flow state is largely affected. In the exhaust region, the fluctuations of pressure, velocity and temperature as well as the average values of velocity are significantly reduced. This illustrates that the spiral outlet can effectively suppress the fluctuations of pressure, thus reducing reflux shock and energy dissipation. In the intake area, the average value of pressure, velocity and temperature are slightly declined, but the fluctuations of them are significantly reduced, indicating that the spiral inlet plays the role in making the flow more stable. The numerical results obtained reveal the three-dimensional flow characteristics of the positive displacement blower with spiral inlet and outlet, and provide useful reference to improve performance and empirical correction in the noise-reduction design of the positive displacement blowers.