Aiming at the requirements of high surface accuracy, high reliability and high stability of large aperture off-axis mirror assembly, a new type of composite support scheme for off-axis mirror was designed, which was supported by central cylinder and four points that could lock or unlock freely. The heat locking parts were locked during mechanical test and launching, released during ground test and on orbit. It could effectively improve the fundamental frequency and the ability to resist vibration, while ensuring the surface quality. In the end, the design was verified in the development of 706 mm×560 mm off-axis mirror assembly of a space remote sensor. The results show that the dynamic performance of the mirror assembly is very excellent when the four auxiliary support points is locking, that can meet the requirements of adhesive stress ≤3 MPa when the fundamental frequency is greater than 120 Hz and the static load is greater than 30 g, and the RMS of the surface of the mirror is better than λ/40 （λ=632.8 nm） when the four auxiliary support points locked or unlocked. The support design is reasonable and effective.