In this paper, the limitation of the Boussinesq equation for generating focusing waves is discussed in terms of the vertical asymmetry factor (VAF) and the water depth. The relationship between them is investigated through experiments in numerical wave tank (NWT) based on both the Boussinesq and fully nonlinear models. A fitting curve associating the VAF and the maximum water depth is suggested based on the numerical results. By using the curve, user could determine whether the Boussinesq model is applicable for studying focusing waves if both the parameters are given.