The finite difference time domain(FDTD) method with high-order approximate was introduced to the time-domain analysis of electrically-large planar optical waveguide devices.The perfectly matched layers(PML) absorbing boundary condition for higher-order FDTD method has been implemented and the numerical dispersion of this scheme was studied.The numerical simulations for the parallel-slab directional coupler were presented and the computing results using this scheme are in highly accordance with analytical solutions.Compared with conventional FDTD method,this scheme could save considerable computational resource without sacrificing solution accuracy and especially could be applied in the computer-aided design and analysis of electrically-large optical devices.