An alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method based on the fast-adaptive-composite grid ADI (FAC-ADI) scheme is introduced to reduce the truncation error generated when ADI-FDTD is derived from the Crank-Nicolson (CN) ADI scheme, and its efficiency is investigated. The relaxation equation and residual equation for FAC-FDTD are constructed by defining the error vector between the traditional ADI and CN FDTD formulas, and the iterative formulas are defined using the iterative schemes. A comparison of the computational efficiency between the FAC-ADI and ADI-FDTD is presented.