The modified perfectly matched layer (PML) conductivity profiles are proposed to improve the stability of the alternating direction implicit (ADI) finite-difference time-domain (FDTD) method with a split-field PML absorber. The amplification matrix of this scheme is derived based on the Von Neumann method. From the stability analysis, it is found that this scheme is unstable at the PML interface and inside the PML regions. The instability of this scheme inside the PML regions can be improved with the modified PML conductivity profile. The theoretical results are validated by means of numerical simulations.