The predictor-corrector-iteration algorithm in the framework of finite difference method was applied to solve one-dimensional version of two-layer Boussinesq equations,creating a numerical wave tank for focusing wave in time domain.A fourth-order composite Adams-Bashforth-Mouton scheme was adopted for time integration.The internal wave method of summing multi-frequency wave components was used to realize wave focusing in computational domain.Numerical experiments were conduct to reproduce the laboratory measurements of Baldock,and the agreements between the computed results and experimental data were good.Further discussions were carried out to investigate nonlinear effect on the computed results of focusing wave by considering different levels of nonlinearity (strong nonlinearity,weak nonlinearity and linearity) in the numerical model.The results show that nonlinearity plays a very important role in accurately modeling focusing wave.The present model with strong nonlinear terms presents the best results,the weakly nonlinearity presents reasonable results,while the linear model presents the worst.