This paper proposes an improved adaptive domain decomposition finite-difference time-domain (IADD-FDTD) method. We apply an adaptive boundary test to the voltage on the boundary. The new algorithm avoids computational error caused by instability of electric field intensity on the test boundaries, and can get more sub-domains than the original adaptive domain decomposition finite difference time domain (ADD-FDTD) and increase computation speed. Analysis results of some 3-D multilayer interconnected structures show that the proposed algorithm is correct and efficient.