The traditional method for measuring the parameters of the transmitter is that the tester measures the parameters according to the test standard and records the test results manually in the case of non-work time. It has some shortcomings, such as low efficiency, inconvenient data storage and higher requirements for testers. In view of the shortcomings, an automatic online method of measuring harmonic distortion of transmitter is proposed. The method calculates parameter values by collecting transmitter data in real time. It is based on the histogram algorithm. According to the cumulative distribution function, discrete distribution of transmitter input and output signal amplitude values are obtained, and then the least squares algorithm is applied to the nonlinear polynomial fitting of discrete values. The feasibility of the method is verified by experimental simulation, and the residual value and test accuracy of different test methods are given.