To satisfy the requirement of high-resolution hyperspectral imager with a large relative aperture and a wide spectral region, and to avoid the difficulties of fabricating the convex grating in the Offner spectral imaging system and the problems of low relative aperture in the modified Czerny-Turner spectral imaging system, a neotype Schwarzschild spectral imaging system using planar grating with a large relative aperture is proposed. Based on the Rowland circle theory of the reflective spherical surface, the astigmatism-correcting method of the Schwarzschild spectral imaging system is analyzed. The initial parameters computing program is written using Matlab software. As an example, a Schwarzschild spectral imaging system operating in 400～1000 nm waveband with relative aperture of 1/2.5 is designed. First, the initial parameters are computed using our Matlab program, and then the ray tracing and optimization for the spectral imaging system are carried out with Zemax-EE software. The analyzed results demonstrate that the size of spot diagram is less than 13 μm in the whole working waveband, aberration correction is realized, and good imaging quality is obtained in a wide spectral region with a large relative aperture, which satisfies the requirement of design specifications, and proves the feasibility of the novel Schwarzschild spectral imaging system. It has extensive application prospect in hyperspectral remote sensing field.