To measure the nonlinearity in laser heterodyne interferometer,a novel method to measure the nonlinearity was proposed.By data acquisition and spectral analysis for the output signal from the measurement aim which is moving in a constant speed,the first-harmonic and second-harmonic of nonlinearity are separated and the ratio between the nonlinearities and measurement signal is obtained.With the model of relationship between the ratio of amplitude and the nonlinearities that we proposed,the nonlinearity is measured accurately.The experimental result shows that the method is satisfied for the measurement of nonlinearity in laser heterodyne interferometer and the highlight is that the method can measure the nonlinearity accurately and easily realize.