Cold-background multitarget compounding system provides infrared targets for hardware-in-the-loop simulation system, in which large aperture reflective mirrors are employed. In this paper, we proposed a combined belt-back structure and designed the flexible connection to solve support mechanism of large aperture mirrors for 100K temperature variations. By Finite Element Method analysis, root mean square (RMS) of optimized mirror was better than λ/40 under self-gravity and 100K temperature variations. By Zernike polynomial fitting, MTF of multitarget compounding system was over 0.5 and RMS spot diameter was less than 0.05mrad. Results demonstrated that the proposed support mechanism was effective and provided analytical data for 1m level mirror in large environment temperature variations.