This paper presents an alternating direction implicit FDTD (ADI-FDTD) algorithm without the Courant-Friedrich-Levy condition restraint for obtaining the solution of the three dimensional scattering problem.The perfectly matched layers are adopted to simulate the infinite space.The modified formulations of the connective boundaries are also given in detail.Compared with the conventional FDTD
the timestep increment is free from the CFL condition.So the total timestep is decreased significantly and the new algorithm is more efficient than traditional FDTD in terms of CPU time.Finally
some numerical results are given to demonstrate the accuracy and the high efficiency of ADI-FDTD.