A proton-conducting polymer, a preparation method thereof, a polymer membrane manufactured from the proton-conducting polymer, and a fuel cell having the polymer membrane are provided. The proton-conducting polymer contains an acid-bearable aromatic group covalently bonded to its polyimide backbone and a proton-conducting acid group attached to the aromatic group. A polymer membrane manufactured from the proton-conducting polymer has greater mechanical strength and ionic (protonic) conductivity than existing sulfonated polyimide membranes and thus improves the performance of fuel cells. In addition, the proton-conducting polymer membrane is low cost, enabling automobile fuel cells to be commercialized. Since the proton-conducting polymer membrane is less permeable to methanol, it can be used effectively for DMFCs without causing cell performance degradation.