To effectively correct the spectral line curvature of a transmission dispersive imaging spectrometer system, this paper proposes a distortion compensation method based on the off-axis slit. Specifically, the variation in the incident angle of the dispersive element with slit length is calculated on the basis of the assumed correction of the spectral line curvature of the central wavelength. Then, the slit is moved away from the optical axis of the collimator, and the target incident angle of the dispersive element is obtained based on the distortion of the collimator. The Smile distortion (spectral line curvature) of the central wavelength is thereby corrected. The Smile distortion and Keystone distortion (color distortion) of the remaining wavelength curve outward, and overall pincushion distortion is observed. Finally, the remaining spectral line curvature is corrected with the distortion of the converging lens. A reflection grating imaging spectrometer is designed by the proposed method, and the spectral line curvature of the system is corrected with the distortion of the collimator on the premise that the various parameters meet the design requirements. A principle prototype is developed on the basis of the above design. The system achieves a maximum Smile distortion of 12.9 μm and a maximum Keystone distortion of 6.4 μm. The test results meet the design requirements of the system and thus prove the feasibility of the proposed method.