The spectroscopic imaging system in a mid-wave infrared imaging spectrometer was designed and developed for hyperspectral remote sensing satellites on geostationary orbit. According to the characteristics of geostationary orbit, index parameters of the optical system were analyzed. It was pointed that such spectroscopic imaging system features long slit, small relative aperture, and compact size. A double-slit Offner spectroscopic imaging system was designed with a spectral range of 3--5 μm, splicing slit length of 49 mm, spectral sampling distance of 50 nm, and F-number of 5.4. The whole machine refrigeration was adopted to improve optical system sensitivity. The analysis results showed that when the working temperature was lower than 160 K, the noise equivalent temperature difference (NETD) reduced to 0.2 K, and an athermal design was performed on the basis of the cooling temperature. Based on the results of optical design, the whole aluminum alloy imaging system prototype was developed. The peak efficiency of a self-developed convex blazed diffraction grating reached 86.4%. The performance of the prototype at room temperature was tested using high order diffraction spectrum of visible and near-infrared light. The test results showed high imaging quality of the imaging spectrometer with slight smile and keystone distortions; in addition, all specifications met the design requirements.