Mounting large radius-to-thickness ratio beamsplitters for cryogenic space application requires a careful trade-off in stiffness and flexibility design. Both the cryogenic wavefront error and the survivability in launch vibration environments should be satisfied simultaneously. A new support structure is designed in this paper. In the design, the difference of thermal expansion coefficient of different structural materials is used to achieve athermal support. At the same time, multistage radial flexible structures are designed at the connection positions of different materials. The support structure achieves both high stiffness and low cryogenic wavefront error. The simulation results show that the first mode frequency is 191 Hz. The wavefront error changes 0.032λ from 293 K to 133 K based on FEM simulation results. After the beamsplitter is assembled, the cryogenic wavefront error and vibration tests are carried out. The wavefront error changes 0.028λ from 293 K to 130 K. The maximum acceleration response reaches 16g_n at the installation position of the beamsplitter assembly. There is no change for the beamsplitter assembly before and after the vibration test. The stiffness, cryogenic wavefront error and environmental adaptability of the support structure of the cryogenic larger radius-to-thickness beamsplitter in this paper meet the application requirements of space cameras.