With the development of the aerospace industry, space missions are becoming more complicated and diversified, and there is a demand for antenna mechanisms with a larger physical aperture. In this paper, a planar deployable mechanism is proposed, which can form a flat reflection surface with a small gap between plates. To this end, a novel large-scale two-dimensional deployable nine-grid planar antenna mechanism is designed. First, two antenna folding schemes and four supporting mechanism schemes are proposed. Through comparison analysis, the antenna configuration scheme with the best comprehensive performance is selected. A kinematic model of the deployable mechanism is established, and its kinematic characteristics are analyzed. Then, the correctness of the kinematic model is verified by comparing the analytical and simulation results of the kinematic model. Subsequently, a finite element model of the antenna is developed. Based on the response surface method, the structural parameters of the support rods of the antenna are optimized, and a set of optimized solutions with lightweight and high fundamental frequency characteristics are obtained. Finally, a prototype of the proposed nine-grid planar antenna is fabricated. The feasibility of the deployment principle and the rationality of the designed mechanism are verified by deployment experiments.