In order to satisfy the application requirements of the large flux and the high stability for the infrared imaging spectrometer, we propose a new type of static no-slit spatiotemporally mixed modulated Fourier transform imaging spectrometer based on the multi-level micro-mirrors. The working process and the generation manner of optical path difference of the system are analyzed. The front imaging system, as an important component of the imaging spectrometer, determines the distribution of the optical path difference, and its performance directly affects the image quality of target object. According to the generation manner of optical path difference of the system, the front imaging system for the telecentric structure in the image space is analyzed and designed. The athermalization design research is carried out by means of the passive optical elimination thermal difference. The result shows that the modulation transfer function curve of each field reaches the diffraction limit in a temperature range of -20-60 ℃. The front imaging optical system has a good imaging quality in the total step height of the multi-step micro-mirrors. At different temperatures, the maximum incident angle on the image surface is less than 0.02° for the principal ray of each field.