A two-degree-of-freedom homodyne grating interferometer displacement measurement system comprises a grating interferometer, a measuring grating, a preprocessing unit and a signal processing unit. The grating interferometer comprises a laser tube, a lateral displacement beam splitter prism, a polarization beam splitter prism, a quarter wave plate, a referencing grating, a refraction component and a four-channel homodyne structure. The system achieves displacement measurement based on grating diffraction, optical Doppler and homodyne signal processing. The grating interferometer outputs optical signals to the preprocessing unit, and the optical signals are converted into electric signals and transmitted to the signal processing unit. When the grating interferometer and the measuring grating do two-degree-of-freedom linear relative motion, the system can output two pieces of linear displacement. According to the two-degree-of-freedom homodyne grating interferometer displacement measurement system, a secondary diffraction principle is adopted to achieve four optical subdivisions, the sub-nanometer and even higher resolution ratio can be achieved, and the two pieces of the linear displacement can be measured. Homodyne signal processing is adopted, the influences brought by the DC component change and the amplitude change can be eliminated, and the two-degree-of-freedom homodyne grating interferometer displacement measurement system has the advantages of being insensitive to environment, high in measurement precision and the like.