The invention, which belongs to the spectral measurement instrument field, provides a spectrograph that has a small structure and a small size and can meet a high resolution ratio requirement. According to the technical scheme, the spectrograph is composed of a dual-port fiber array, an off-axis parabolic mirror, a plane diffraction grating, and an achromatism quarter-wave plate. A to-be-measured optical signal enters through an upper port of the fiber array; after collimation by the off-axis parabolic mirror, the signal is reflected to the plane diffraction grating; diffracted light perpendicular to the achromatism quarter-wave plate among the diffracted light from the plane diffraction grating is reflected by the rear surface and is returned to the plane diffraction grating according to an original path; diffracted light processed at the second time passes through the off-axis parabolic mirror and then is focused at a lower port of the dual-port fiber array; and then the light enters a receiving fiber and is detected by a detector. With rotation of the achromatism quarter-wave plate, light from different diffraction angles is detected respectively at different times to form spectrum scanning. The technical scheme is is mainly used for designing and manufacturing a spectral measurement instrument.