Abstract: With the use of cnoidal wave theory, the theoretical solutions to shallow water wave diffraction by a V-shaped breakwater are derived, so the existing Airy small amplitude wave theory is effectively extended. By calculating the shallow water wave forces on the V-shaped breakwater, the law of cnoidal wave action on the breakwater has been revealed. The results show that the maximum dimensionless wave forces on the breakwater are obviously larger than those predicted by small amplitude wave theory under the same shallow water conditions. The reflected wave theory on the infinite long vertical thin-walled breakwater is effectively extended by using the method in the paper. The variation of incident wave angle, breakwater angle and ratio of breakwater arm length and water depth may have some related influence on wave effects and the practical maximum diffracted wave forces will increase as the shallow water wave characteristic parameter value increases.