A modified Dyson spectral imaging system was proposed to overcome the problems that the focal plane detector is hard to be arranged in a classical Dyson spectral imaging system. Firstly, the optical design method of the modified Dyson spectral imaging system was developed based on the Rowland circle theory of refractive spherical surface. Then, the initial parameter computing program was programmed using MATLAB software. As an example, a Dyson spectral imaging system operating in 200-1 000 nm with a relative aperture 1/2 was designed. The initial parameters were computed using proposed MATLAB program, and the ray tracing and optimization for the spectral imaging system were performed with ZEMAX-EE sofware. The analyzed results demonstrate that the Root Mean square(RMS) of a spot radius is less than 4.2 μm in the whole working waveband (from 200 nm to 1 000 nm), which implements astigmatism correction and obtains a good imaging quality in a wide spectral region and a large relative aperture. These results prove the feasibility of the optical design method proposed.