A practical flapping wing micro aerial vehicle should have ability to withstand stochastic deviations of flight velocities. The responses of the time-averaged thrust coefficient and the propulsive efficiency with respect to a stochastic flight velocity deviation under Gauss distribution were numerically investigated using a classic Monte Carlo method. The response surface method was employed to surrogate the high fidelity model to save computational cost. It is observed that both of the time-averaged thrust coefficient and the propulsive efficiency obey a Gauss-like but not the exact Gauss distribution. The effect caused by the velocity deviation on the time-averaged thrust coefficient is larger than the one on the propulsive efficiency.