An athermal mid-infrared imaging spectrometer based on the Offner scheme was designed to meet the requirement of mid-infrared spectrum analysis. According to the solution of the athermal equations, athermal structure could be designed reasonably through reasonable combination of materials with different thermal properties. Method of solving the initial structure and optical software optimization was introduced based on the analysis of Offner structure principle. A push broom imaging spectrometer with 3-5 μm spectral range was designed by this method, and its operating temperature range is from -40 ℃ to 100 ℃ depending on the achromatic optimization. The spectrometer has good spectral performances, the dispersion spot diameter of which is less than 30 μm in the full field of view. The spectrometer′s transfer function value is greater than 0.35 at the spatial cutoff frequency of 16.7 lp/mm, and its spectral resolution is less than 15.6 nm.