Concerning the phenomenon of tidally-induced groundwater fluctuation in a coastal aquifer, based on the new Boussinesq equation derived by Kong et al., the function express and composition of the wave were analyzed using the perturbation method. Analytical research on the influence mechanism of a tidally-induced high-frequency groundwater wave affected by the unsaturated zone aquifer was conducted. The results indicate that, after the groundwater wave propagated into the coastal aquifer, the dominant frequency amplitude decreased exponentially, and the smaller the hydraulic conductivity was, the larger the decay rate of wave energy was. Besides, under the nonlinear effect, a high-frequency wave was generated during the wave propagation process, and the wave energy transformed from a low-frequency wave to a high-frequency wave. Numerical experiment results show that the increase of unsaturated water holding capacity, the decrease of thickness of the unsaturated zone, and the decrease of porosity of soil promote the formation of a high-frequency wave.