To achieve high-precision measurement of the all-Stokes vector, a mutual differential frequency modulation approach for dual photo-elastic modulators (photoelectric modulator, PEM) in all-Stokes vector measurement was proposed. The incident light was differently frequency modulated by two PEMs of different frequencies. The measured polarization vector and the phase delay amplitude of the PEM were simultaneously modulated at different differential frequency components. The phase delay amplitude of the PEM was obtained in real time by dividing the odd differential frequency components, and the Stokes vector of the measured light was accurately obtained by combining different differential frequency components. This method reduced the error introduced by the fluctuation of the phase delay amplitude in the PEM measurement system. Theoretical and experimental analyses were conducted, and the results showed that the variance of the measured Stokes vector was 10^?5. This method will provide support for high-precision polarization measurement.