On the basis of characteristics of optical systems of Offner spectral imagers, the material choices of every parts, structure design of mirrors and frames in the imagers were investigated, then the structure design of a compact Offner spectral imaging system was achieved and its 3D model was established. In order to prove the reasonability of the structure, the 3D model was analysed by finite element method. The analysis results indicate that the first natural frequency of the structure is 362 Hz, which is higher than 120 Hz requested by the system. Furthermore, the analysis results of weight and temperature indicate that the RMS of surface figure, distances, centrifugal axes and the deflection of angle in every important parts all meet the requirements of the system tolerance. These results validate the rationality of structure of the compact Offner spectral imaging system.