The invention discloses a pulse delayer and a delay method applied to quantum key distribution. The device comprises an optical pulse generation module and an optical pulse delay module. The optical pulse generation module is used for outputting optical pulses with consistent polarization directions to the optical pulse delay module; and the optical pulse delay module is used for splitting the input optical pulse, controlling the cycle index of the optical pulse in the optical pulse delay module by adjusting the phase of the split optical pulse, and outputting the delayed optical pulse when the required delay time is reached. The loop-back differential phase shift quantum key distribution system has the advantages that any loop-back frequency of the optical pulse in the optical path is actively controlled by splitting the optical pulse and adjusting the phase of the optical pulse, so that the delay time of the optical pulse is actively controlled, and the loop-back digit of the loop-back differential phase shift quantum key distribution system is improved; the error-tolerant rate of the key distribution protocol is increased while the protocol requirement is better met, and the maximum information which can be obtained by an eavesdropper is reduced.