Phase retardation is an important indicator of polarizing optical elements. To measure the phase retardation of the polarizing element accurately and quickly, a method with phase compensation and cascade modulation for detecting the phase retardation of the polarizing elements is proposed. In this method, a photo-elastic modulator (PEM) and an electro-optical modulator (EOM) are used as the cascade modulation element of the phase delay detection system, and a Soleil-Babinet phase compensator is used to compensate phase retardation for the sample. Using digital phase lock technology and a field-programmable gate array (FPGA)-based on-chip programmable system, it detects the phase parameters of the Soleil-Babinet phase compensator corresponding to the extreme point of light intensity and performs data processing to realize the detection of the phase retardation of the sample. Experiments show that the maximum relative error of using this method to measure the phase delay of a sample is 0.857% and the measurement accuracy is 99.143%, which verifies that the combination of the polarization modulation and compensation methods to measure the phase retardation has high accuracy, and the influence of the compensator itself on the measurement error is reduced.