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In this paper, based on quadratic interpolation in time domain, an improved first-order absorbing boundary condition (ABC) for the alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method is developed. The ABC is derived in two half-time steps iteration form which is very suitable for the ADI-FDTD method. The reflection of the ABC caused by both the truncated error and the phase velocity error is analyzed in detail. The phase velocity estimation and nonuniform cell are adopted to improve the performance of the ABC. Numerical results show that efficient improvement on the absorbing performance is achieved based on these methods.