The invention relates to a method and a system for transmitting and receiving by orthogonally dual-polarized differential quaternary phase shift keying. The system comprises a transmitting device and a receiving device, wherein the transmitting device comprises a continuous laser; the continuous laser is connected with a polarization beam splitter which is connected with a DQPSK modulator; the DQPSK modulator is connected with a precoder of which the output end is connected with a tunable optical attenuator or a polarization controller; and the polarization controller is connected with a polarization beam combiner; and the receiving device comprises a DQPSK demodulator, and the DQPSK demodulator is connected with a balanced detector which is connected with a decision circuit. In the method, four paths of electrical signals can be multiplexed, and only one demodulator and two balanced detectors are used at a receiving end to demodulate two orthogonally polarized DQPSK signals at the same time; therefore, compared with D8PSK and x-QAM systems, the system has the advantages of greatly reducing the complexity and remarkably enhancing the stability of components.