In certain conditions，shallow water waves may emerge in solitary waveform，causing wave effects on offshore structures. Based on diffracted wave theory，we derive the theoretical solutions to first-order solitary water wave diffraction by a V-shaped thin wall breakwater，and study the action rule of a solitary wave on the breakwater by calculating solitary diffracted wave forces on the V-shaped breakwater. By calculating diffracted wave force on the finite length vertical thin-walled V-shaped breakwater with the opening angle 180° while the finite length breakwater is enough long，we carry out effectively similarity comparison with the reflected wave force caused by a solitary wave on the infinite long vertical thin-walled breakwater. The results show that the maximum dimensionless wave forces on the breakwater caused by solitary wave are obviously larger than those predicted by small-amplitude wave theory under Airy same shallow water conditions，reflecting the nonlinear effect of shallow water wave. The variation of incident wave angle，opening angle of V-shaped breakwater，the ratio of breakwater arm length and water depth，and solitary wave characteristic parameter may have some influence on wave loads.