Introduce the fundamental suppression method and spectrum analysis method for distortion measurement. Discuss the limitation of the two methods for superlow distortion measurement. Propose an improved method for Superlow distortion measurement, using the fundamental suppressor attenuate fundamental before signal enters the spectrum analyzer to satisfy the dynamic range. According to the fundamental suppression and each harmonic residual amount of distortion level, use the derived formula to calculate the distortion and validate the measurement results. This method combines the advantages of fundamental suppression method and spectral analysis method, can effectively reduce the lower limit of measurement range to -130 dB and improve the accuracy of measurement.