The classic Offner spectrometer has the advantages of having a simple structure, light weight, and high imaging quality. It is easily to achieve a fixed spectral resolution but cannot meet usage requirements. Therefore, we present a practical method for designing a spectrometer with variable spectral resolution. Multiple off-axis convex (OAC) gratings are used to replace the convex grating in the classic Offner spectrometer. We derive the principle through ray tracing and establish an optimization process for the basic parameters of multiple OAC gratings. To demonstrate this method, a corresponding system is designed. The results show that a variable spectral resolution, with a variation ratio close to 4, of 0.45-1.91 nm is achieved over a wide bandwidth of 460-900 nm. Additionally, the smile and keystone of the system are well corrected.