Abstract: Shore protection structures such as detached breakwaters may cause wave diffraction and refraction, and change the hydrodynamics in coastal region. An extended version of XBeach model is presented in this paper to improve the accuracy of prediction on wave, current fields and sandy beach evolution when wave diffraction takes effect. The extended model is validated by comparing the simulation results with the physical test of wave propagation through a breakwater gap, with satisfactory agreement. Numerical simulation test was carried out for an ideal flat beach with a slope of 1∶50 behind a breakwater and results show that the extended model generates more reasonable results. Based on the extended XBeach model, the evaluation system of the protection effect is established for the offshore breakwater group considering the engineering quantity. The possible effects of width and height of breakwaters on the evolution of plane sandy coast are analyzed under certain wave condition based on numerical tests leading to the optimal scheme finally presented. The proposed index is different from previous researches on the morphological evolution of sandy beaches behind the breakwaters, and has high practical engineering value.