The image quality of the traditional Offner spectrometer may be unsatisfied when large dispersion is needed. Furthermore, severe ray obstruction may occur when relative aperture is not small enough. Therefore, an off-plane Offner spectrometer is suggested based on Rowland condition in this paper. An analytical design approach of an imaging spectrometer based on off- plane Offner configuration is presented. Astigmatism and coma are corrected in the design without ray obstruction. An imaging spectrometer is designed using the configuration with a spectral range from 350 nm to 1000 nm. The size of the spectral image is 12.6 mm. The modulation transfer functions in total fields of view and the whole spectral range are more than 0.78. The root mean square (RMS) spot radiuses are smaller than 4 μm. Meanwhile, the smile and keystone of the system are both better than 1% of a pixel size. Finally, a contrast between the off-plane Offner spectrometer and the in-plane configuration is performed. The result demonstrates that the off-plane Offner spectrometer has a better image quality for high spectral resolution if a shot entrance slit is allowed. There is also an advantage in eliminating the smile and keystone. The configuration can be applied to imaging spectrometers with small dimension and high spectral resolution.