The invention relates to the technical field of space optical remote sensors, in particular to a frame structure of a small off-axis three-counter-ionospheric imager, which solves the problem of low rigidity of the frame structure caused by the large aspect ratio of the structure caused by the long focal length and wide field of view of the optical system. Insufficient stray light suppression mainly includes the front cover, rear cover, off-axis three-mirror assembly, main frame, detector assembly, T-shaped slit structure, truss rod, stray light elimination trap, light shielding tube I, light shielding Cylinder II and detector aperture. The off-axis three-mirror assembly includes a primary mirror assembly, a secondary mirror assembly, and a three-mirror assembly; the main frame includes a light entrance plate, a primary mirror substrate, a secondary mirror substrate, a three-mirror substrate, a detector substrate, a top plate, and a bottom plate , the supporting plate I, the supporting plate II, the light blocking plate I, the light blocking plate II, and the light blocking plate III. The invention has high structural space utilization rate and high specific rigidity, can suppress the stray light of the off-axis three-mirror system in the whole path, and effectively improve the imaging quality of the optical system.