The linear polarization measurement technology based on spectral modulation can modulate the polarization information of incident light to the spectral dimension through the spectral modulation module. The spectral modulation module is composed of a achromatic quarter-wave plate, a multiple-order wave plate and a polarization beam splitter, which can obtain the linear polarization and spectral information of the target in a single measurement. With the combination of modulation module and grating spectrometer, a dual-channel polarization measurement system is designed. The polarization measurement model of the system is derived, the influence of spectral broadening on the modulation spectrum is analyzed, and the partial periodic least squares curve fitting method is used to demodulate the polarization information. Moreover, a test device is built to verify the performance of the measurement system. First, the complete linear polarization light is used to calibrate the retardation of the multiple-order wave plate and the polarimetric efficiency of the system. Then, the polarization measurement accuracy of the system is verified by using the variable polarization light source. The experimental results show that maximum absolute deviation between the theoretical value and the measured value of degree of linear polarization of the variable polarization light source is 1.11%, and the maximum deviation of angle of linear polarization is 0.7°, which means the proposed system has high polarization measurement accuracy.