The pulsation in the exhaust process of three-lobe involute type Roots blower was simulated in CFD method under the compressible、unsteady condition. To achieve the change of exhaust area that dynamic mesh updating method and multiple reference frame at rotor-stator coupled area were adopted.The result of numerical simulation was contrasted with computing result of the semi-experience theory formula.It was discussed that the characteristic of air flow fluctuation in a period and influence to the exhaust flux.