Disclosed is a symmetric-type grating heterodyne interference secondary diffraction measuring device. The device mainly comprises a laser device, a 1/4 wave plate, a light splitting system, a photoelectric conversion system, a polarizing light splitter, a reflector, a grating and the like. Double-frequency lasers emitted by the laser device enter the light splitting system after passing the 1/4 wave plate to be divided into three branches, one branch enters the reference photoelectric conversion system, the other two branches enter the positive-negative measuring light path polarizing light splitter, reflected light s is irradiated onto the grating by an incident angle set according to corresponding diffraction grades, occurs interference with light p on a polarizing light splitting face after secondary diffraction and enters the measuring light path photoelectric conversion system. When the grating moves, two signals containing displacement information are detected by the measuring light path photoelectric conversion system, and grading level and vertical displacement can be acquired by solving through the two signals. Whether measured light can return along an original path after secondary diffraction or not only depends on the incident angle and has nothing to do with grating displacement, so that the symmetric-type grating heterodyne interference secondary diffraction measuring device can realize horizontal long stroke and vertical displacement measuring.