Abstract: In laser Doppler velocity measurement system based on linear frequency sweep mode, laser irradiates the moving target, which causes the change in laser frequency, the change is called Doppler frequency offset. Doppler frequency offset can be obtained from the mixture of the local vibration of laser and the returned laser from the target, and then the velocity can be calculated based on the offset. However, the nonlinearity in frequency sweep deeply impacts the accuracy of velocity measture. First, the effects of the nonlinearity in frequency sweep was simulated, the nonlinearity in frequency sweep could deeply affect the accuracy of the velocity measurement and caused the error of polarity, the effect was velificated. And a method was proposed to correct the nonlinearity and it was verified through the experiment. This will improve the the accuracy and reduce the error in laser Doppler velocity measurement.