Disclosed herein is a composite-structured organic semiconductor polymer for an organic thin film transistor which contains quinoxaline rings in the backbone of the polymer. According to the organic semiconductor polymer, since quinoxaline rings having n-type semiconductor characteristics, such as high electron affinity, are incorporated into a polythiophene having p-type semiconductor characteristics, the organic semiconductor polymer simultaneously exhibits both p-type and n-type semiconductor characteristics. In addition, the polythienylquinoxaline derivative exhibits high solubility in organic solvents, co-planarity and stability in air. Furthermore, when the polythienylquinoxaline derivative is used as an active layer of an organic thin film transistor, the organic thin film transistor exhibits a high charge carrier mobility and a low off-state leakage current.