To evaluate the insulation condition of power cable, a new method to calculate equivalent resistance value of and evaluate insulation condition of cable was presented under the impulse voltage. To simulate the circuit parameters of the components and the whole layer of power cable, the RC series and parallel equivalent circuit of power cable was built. The mathematical expression of the equivalent circuit parameters of the power cable was given by using the electromagnetic field theory, and the method to calculate the equivalent resistance of power cable was proposed, and the calculation expression of the equivalent resistance of power cable under impulse voltage was derived, and a new method for evaluating the insulation condition of power cable was summarized. The equivalent resistance of cable was simulated and calculated by ATP and MATLAB software; and the simulation and calculation results show that the maximum error between the setting value of resistance and the calculating value of the equivalent resistance is 0.4%, and the reliability of the new method was verified.