Disclosed is a power transformer insulation aging state assessment method based on a frequency domain Cole-Davidson model. The method comprises the following steps: 1), carrying out wiring according to instrument instructions; 2), setting parameters; 3), ending testing, and obtaining a frequency domain characteristic parameter fp through spectrum solution; 4), according to the frequency domain characteristic parameter fp and an oil paper insulation temperature T, introducing into a specified formula, and calculating and determining an insulation aging state, i.e., a DP value; and 6), when the average polymerization degree of oil impregnated paper is deceased to 500, determining that the overall insulation of an oil paper insulation electric power device is at a mid-life state; when the average polymerization degree is decreased to 250, determining that the life of the oil paper insulation electric power device is already terminated; and when the polymerization degree is decreased to 150, determining that the mechanical strength of the insulation paper is zero. According to the invention, core suspension is eliminated, insulation materials are not damaged, sampling is unnecessary, the insulation paper average polymerization degree DP value can be accurately assessed, and the oil paper insulation aging state is determined.