Conventional imaging spectrometers generally have low variable ratio, which is not conducive to the extended application of large-field, long-slit, multi-channel optical systems. In space remote sensing, the radiation energy of the ultraviolet band is low, which requires the imaging spectrometer to have a smaller F-number. In order to meet the requirement of detecting small F-number of high spectral resolution imaging spectrometer, an Offner UV imaging spectrometer with high spectral resolution and high variable ratio is designed in this paper. An improved Offner structure with light and small size is adopted in the rear beam splitting system of the imaging spectrometer. Based on the requirements of variable power ratio and small F-number of the imaging spectrometer, the initial Offner structure parameters are derived theoretically. A meniscus lens is inserted in front of the image to increase the degree of freedom for the optimization of the system and improve the imaging quality of the system. The obtained imaging spectrometer works in the 270~300 nm band with a long slit of 40 mm, a spectral resolution better than 0.6 nm, the system variable power ratio less than 0.22, and an F number less than 2. Its Modulation Transfer Function (MTF) is better than 0.9 at a cutoff frequency of 14 lp/mm, and the Root Mean Square (RMS) radius of each field of view in each band is less than 12 μm. This study provides a design scheme for the UV-band hyperspectral detection imaging spectrometer with small F-number and high variable ratio.