Polymer membranes are attractive mirror candidate for the space large aperture lightweight optical imaging system. But there are strict requirements for mirror material because of the harsh space application environment and the high optical imaging quality requirement. The dimensional stability is one of the most important properties for optical mirror material. In this research, based on the molecular structure design, rigid molecular chain and hydrogen chain have been introduced to polyimide to improve the thermal dimensional stability. At the same time, the excellent mechanical, optical and thermal properties of the polyimide membrane have been guaranteed. The obtained optical grade polyimide has high dimensional stability, and the optical stability of the space environment and the excellent comprehensive properties are good candidates for the lightweight optical application.