The invention provides a homodyne one-dimensional grating displacement measuring device, comprising a single-frequency laser, a polarization beam splitting prism, a beam splitting prism, a plane mirror group, a quarter wave plate, two signal receiving units and a signal processing system, wherein the single-frequency laser It is used to emit a beam of linearly polarized light with a fixed frequency, which is incident to the polarizing beam splitter prism for beam splitting, and a beam is reflected to the plane mirror group, which is vertically incident to the diffraction grating through the 1 / 4 wave plate, and diffracted by the diffraction grating to generate diffracted light, which is passed through the plane mirror group, The 1 / 4 wave plate and the polarizing beam splitting prism are incident on the beam splitting prism; the other beam is symmetrical with it to generate diffracted light, and the two diffracted beams interfere to form the interference light after being split by the beam splitting prism and enter the two signal receiving units respectively. The invention can not only realize the straightness measurement of large stroke, but also realize the displacement measurement of four times subdivision of single diffraction, and can realize the displacement measurement of eight times subdivision by using right angle prism or corner prism.