The synthetic subdomain method (SSDM) of finite-difference time-domain (FDTD) method is used to simulate the large-scale optical waveguides. The waveguide is divided into two or more parts, and the parts whose influence on the simulation result is ignorable are deleted on the condition that the simulating precision does not deteriorate. In the simulation, the absorption boundary condition is changed accordingly, and the light transmission and loss are also considered. The simulation results agrees with that of conventional method, and no precision decline is found. The two-part method occupies 55% memory and 60% time of the conventional FDTD method. Three-part SSDM uses 31% memory and 28%. It is demonstrated that the SSDM is superior to the conventional FDTD method for large-scale integrated optical waveguide simulation, saves hardware resource and time, and improves the working efficiency.