A differential quadrature phase shift keying (DQPSK) system, method, and device are disclosed. The DQPSK system includes: a transmitter, configured to: pre-code an input first signal and second signal, and generate an in-phase signal and a quadrature signal; modulate the in-phase signal to generate a first differential phase shift keying (DPSK) signal, and modulate the quadrature signal to generate a second DPSK signal; perform a 90 degree phase shift on the first DPSK signal or the second DPSK signal, and interfere with the other DPSK signal to obtain a DQPSK signal; and send the DQPSK signal to a receiver; the receiver, configured to: demodulate the DQPSK signal sent from the transmitter in detuned filter mode, and restore the first signal and the second signal through optical/electrical (o/E) conversion. With the present invention, the accurate control on the phase difference between two arms of an Asymmetric Mach-Zehnder Interferometer (AMZI) is avoided, thus facilitating the signal control and adjustment, and greatly lowering the system cost.