In order to optimize the self-powered supplies on transmission lines, including reducing the volume of the energy power coil and improving its economy, the optimal number of coil turns for self-powered supply is studied. Based on the Jiles-Atherton hysteresis theory, the hysteresis characteristics of the energy power coil are simulated to select the suitable coil material. Furthermore, the calculation method for the optimal number of coil turns, which can make the coils stably work in the unsaturated region, is proposed. Considering installation convenience, open-close coil is used and the influence of its air gap width on the optimal number of coil turns is discussed. The calculation results show that air gap causes the decrease of optimal turn optimal number of coil turns. However, power loss caused by air gap increases significantly with the increase of air gap width, so the air gap width should be reduced as much as possible. Finally, the effectiveness of the design scheme is verified by simulations and experiments.