Abstract: This paper aims to propose a method for judging the critical commutation failure voltage in accordance with the transient response characteristics of DC current. Firstly, the effects of DC line reactance and charging capacitance on DC current are studied. Secondly, A direct current transient prediction algorithm after fault of AC system is proposed based on an equivalent model of DC transmission lines under Laplace Space established in this paper. The inverse Laplace transformation method of Stehfest algorithm is used to get the time domain expression of the DC current in the fault process, and the criterion of the instantaneous voltage value of the commutation failure is derived. Simulation results under different operating parameters verify the correctness of the proposed DC prediction algorithm and the accuracy of the proposed criterion.