The utility model provides an optical pulse repetition frequency expander and a laser application system which are suitable for the photoelectric field. The optical pulse repetition frequency expander is a cascaded optical fibre coupler formed by a plurality of couplers sequentially connected, wherein each of the couplers is provided with an input terminal, an output terminal and two connecting arms; and delta LN that the length of one connecting arm is compared with the length of the other connecting arm is equal to c divided by 2 multiplied by Nfn, wherein N is the cascade number of the cascaded optical fibre coupler, and n is the refraction index of a fiber core. In the embodiment of the utility model, the optical path difference is introduced in one connecting arm of the cascaded optical fibre coupler to design the fully fiberoptic optical pulse repetition frequency expander with simple structure and low cost. The optical pulse repetition frequency expander can expand the repetition frequency of the current pulse laser commonly used and can especially expand the repetition frequency of the picopulse laser with the repetition frequency MHZ and the millimicrosecond pulse laser with the repetition frequency KHZ for multi-stage frequency doubling expansion, so as to be used for improving the power output level of the current pulse optical fiber laser.