In this work, the microstructure of composite electrolyte is reconstructed by randomly packing cubes, and the charge transport process is simulated to calculate the effective conductivity ratio sigma(eff)/sigma(0) of the designated conducting phase. Effects of the volume fraction and the electrolyte particle size on sigma(eff)/sigma(0) are also examined. Results show that sigma(eff)/sigma(0) is independent of the particle size but sensitive to its volume fraction. Finally, a new formula is proposed to quickly predict the sigma(eff)/sigma(0) of the designated conducting phase in composite electrolyte with varying compositions.