Numerical simulation on the separation flow control over a bump was carried out with synthetic jet actuator before the separation point. The Reynolds averaged Navier-Stocks equations were solved by FLUENT® 6.3. The mechanism of synthetic jet actuator flow separation control was discussed by analyzing the friction coefficient, the vortex structure separated from the bump and the flow field near the actuator orifice. When the synthetic jet actuator was turned on, the length of the recirculation region can be reduced, and the separation of flow was locked by synthetic jet, so the frequency of separation was equal to the frequency of synthetic jet. For the fixed actuating frequency, the larger the blowing momentum coefficients of the synthetic jet,the shorter the separation region downstream of the bump. When the maximum blowing momentum coefficient is 0.369 1%, the length of separation region can be reduced by 11%.