LFM is one of widely used methods of ISAR. Nonlinearity in frequency sweep has some effects on the quality of imaging. Especially in the high-frequency, broad-band and short-tune frequency sweep, frequency swept nonlineatity of LFM curve can hardly distribute it. Based on the theory of ISAR, using simple maths methods, the method of using arbitrary times polynomial simulation for the nonlinearity testing of LFM signal is studied for the first time. Finally, the correctness of this method is approved by one experiment.