The invention discloses a double-frequency grating interferometer displacement measurement system. The system comprises a double-frequency laser device, an interferometer, a measuring grating and an electrical signal processing portion. The measurement system achieves displacement measurement based on optical grating diffraction, the optical doppler principle and an optical beat frequency principle. The double-frequency laser device transmits double-frequency laser, the laser is divided into reference light and measuring light through a polarizing beam splitter, the measuring light transmits into a position of the measuring grating, positive and negative first-order diffraction occurs, diffraction light and the reference light form a beat frequency signal which contains displacement information in two directions at the position of a light detection unit, and linear displacement output is achieved through signal processing. According to the system, the sub-nanometer-grade or even higher grade of resolution ratio and accuracy can be achieved, and horizontal large-stroke displacement and horizontal displacement can be measured at the same time. The system has the advantages of being insensitive to the environment, high in measuring accuracy, small in volume and light in weight. The system serves as a lithography machine ultraprecise workpiece platform position measurement system, the comprehensive performance of the workpiece platform can be improved.