The effect of background fluctuation on velocity diagnostics is discussed and studied.The kinetic theory of Mach probe (MP) and the theory of BGK mode are combined to evaluate how the measurement of MP is affected by electrostatics fluctuation.It is found that the quantity of speed by the MP model is closer to the effective velocity in the picture of momentum flux rather than the real mean velocity,while,with high fluctuation,the fitting parameter of MP's exponential formula should be corrected.