The invention relates to the field of raman spectrometer, in particular to a medium-stepped grating type spatial heterodyne raman spectrometer optical path structure. Two middle-step gratings (or onemiddle-step grating and one plane reflector) are adopted, and a spatial heterodyne interference effect is generated to realize spatial heterodyne raman spectrum measurement, so that the problems of low measuring efficiency, low resolution and small visual field broadening angle due to the fact that a moving part is generally arranged in a traditional fourier transform raman spectrometer are solved. The use level N of the middle-step grating is larger than 1. Under the condition that the range of the total measuring wave band of the raman spectrum of an optical path remains unchanged, the morethe use level N of the middle-step grating is, the higher the raman spectrum resolution of the optical path structure is. The back scattering raman spectrum of a sample can be measured, and the transmission raman spectrum of the sample can also be measured.