A new method for precisely measuring mid-infrared waveplate phase retardation based on dual photoelastic frequency difference modulation is proposed. The modulation frequency of the waveplate measurement system is reduced by the frequency difference between two ZnSe dual-photoelastic modulators (PEMs), and low-frequency modulation signal carrying the tested retardation is generated. The phase retardation of the tested waveplate is obtained by dividing the 1-time frequency difference amplitude and 2-time frequency difference amplitude after modulation. The effect of light intensity fluctuations and PEM phase retardation fluctuations on the measurement precision of the system is effectively suppressed by the proposed method and the measurement precision is improved. The measurement principle is deduced theoretically. The ZnSe-PEMs and the experimental system are designed. The experimental results show that the relative error of phase retardation measured is better than 0.004% and the sensitivity can be up to 5×10 -4 rad.