Under the action of high-order harmonics, the potential distribution of the multi-layer parallel winding of the dry-type air-core smoothing reactor will form abnormal distortion, resulting in local field strength concentration and accelerated electrical aging, which will affect the service life of the reactor. To solve this problem, firstly, based on Fourier analysis, the high-order harmonic characteristics of a smoothing reactor under operating conditions are obtained. Then, the physical model of the reactor with the line turn as the basic unit is established, and the potential distribution law of the dry-type air-core smoothing reactor winding under high-order harmonics and direct current (DC) superposition is studied by the finite element analysis method of field-circuit coupling. The results show that compared with the DC case, the loading of high-order harmonics will make the electric field distribution of the reactor winding uneven, and the field strength of the strongest point in the overall electric field of the reactor increases by 15%. Under the influence of high-order harmonics, the inter-turn field strength difference between the upper and lower ends of the single-layer winding of the reactor increases, and the inter-layer field strength of the windings on both sides of the encapsulation increases. The maximum electric field of the outermost winding is about 20% higher than that of the innermost winding. As a result, the outermost encapsulation of the dry-type air-core smoothing reactor is the most prone to electrical aging.