Abstract: To address the urgent demands for large-scale, high-packing-ratio, and light-weight sunshade in high-resolution imaging and multi-band observation missions of space optical equipment, a secondary variable-dimension deployable sunshade mechanism was proposed. Firstly, based on the modular design concept, the overall scheme design of the deployable sunshade mechanism was carried out. Secondly, leveraging the variable-cross-section triangular pyramid foldable-deployable mechanism and the Sarrus mechanism, the circumferential and axial deployable mechanisms were designed. Thirdly, kinematic analyses of the circumferential and axial deployable mechanisms were conducted respectively by using the screw theory and the node coordinate method, and the motion characteristics of key points within the mechanism were investigated. Finally, Adams-based dynamic simulation and scaled prototype testing were performed on the integrated mechanism to validate the correctness of the structural design. The research results indicate that the proposed sunshade mechanism can achieve variable-dimension deployment with a coordinated and interference-free deployment process, confirming the validity of the structural design scheme and its underlying principles. This study provides valuable insights and references for the development of similar sunshade mechanisms.