Disclosed herein are a polyimide-polybenzoxazole copolymer, a method for preparing thereof and a gas separation membrane comprising the same. More specifically, provided are a polyimide-polybenzoxazole copolymer simply prepared through thermal-rearrangement performed by thermally treating a polyimide-poly (hydroxyimide) copolymer as a precursor, a method for preparing the same, and a gas separation membrane comprising the same. The copolymer shows superior gas permeability and gas selectivity, thus being suitable for use in gas separation membranes in various forms such as films, fibers or hollow fibers. The gas separation membrane thus prepared can advantageously endure even harsh conditions such as long operation time acidic conditions and high humidity due to the rigid polymer backbone present in the copolymer.