To study the adverse effect of wavebreak forest on river flood, a numerical model was constructed using MIKE21 FM model, and the wavebreak forest was described by the structure to build the numerical model. Outputs including water level before the dike, average cross-section water level and cross-section velocity were quantitatively compared between conditions with and without wavebreak forest to analyze the effect. The Dongyang Dike of Nenjiang River was taken as an example, and results show that the wavebreak forest influences the changes of water level and velocity. The wider the forest, the greater the water depth, and the greater the influence of wavebreak forest on the flood discharge. Meanwhile, the effect of wavebreak forest on flood discharge capacity is very small under current condition and designed condition on the Dongyang reach.