In this study, we propose a design method of advanced Offner-type convex grating spectral radiation calibration light source optical system to solve the problem of low energy output in case of digital tunable light sources. Further, we theoretically deduce the relation between slit and astigmatism of the Offner-type spectral imaging structure based on the theory of ray tracing. The residual astigmatism in the large slit of the Offner-type spectral imaging system is compensated using a double cylindrical lens. Using the proposed method, a traditional Offner-type spectral imaging system with a slit length of 0.4 mm and an advanced Offner-type spectral imaging system with a slit length of 8 mm are designed having the same spectral range from 500 to 800 nm. The results indicate that the advanced Offner-type spectral imaging system exhibits good image quality and that the root mean square (RMS) radius of full field of view spot diagram is less than 8.1 μm; the RMS radius along the Y direction is less than 6.7 μm, which is within the size of a single pixel. The smile is 6.2% of the size of a single pixel, whereas color distortion is 5.8% of the size of a single pixel; thus, spectral overlapping and spectral offset are diminished. Therefore, the proposed method has some research significance and engineering value to ensure the improved precision of spectral radiometric calibration in case of remote sensing instruments.