Provided is a novel aromatic polyester-based polymer. The aromatic polyester-based polymer includes the repeating unit represented by the following Chemical Formula I and contains at least one aromatic diol monomer. The aromatic polyester-based polymer has a high glass transition temperature and excellent heat resistance, mechanical properties and transparency, as well as shows excellent formability and processability into films or the like, and thus solves the problems occurring in the conventional crystalline or amorphous engineering plastic materials. In addition, the aromatic polyester-based polymer may be applied to next-generation engineering plastic composite materials, such as matrix resins of composite materials for cars and airplanes. The aromatic polyester-based polymer may also be applied to flexible substrates for next-generation flexible displays, solar cells or electronic paper requiring transparency and high heat resistance at the same time.