For the power supply problem of monitoring devices on long distance high-voltage transmission lines, this paper constructs an implementation framework of AC power source based on the current transformer (CT). First, the current transformer adding the third auxiliary winding is adopted in framework, and the power electronic circuit and a new control algorithm are used to control the state change of the third auxiliary winding, that is, when the auxiliary winding is shorted, the CT load winding voltage is zero, while the auxiliary winding is open, the CT load winding voltage output is recovered, so this control strategy modulates the amplitude of the designed power source output voltage. Then, a controller was designed for stabilizing power source output voltage and a new control algorithm considered the excitation nonlinearity of CT was proposed based on the least-squares curve fitting method. Finally, based on the simulation model established with Matlab/Simulink, the feasibility of designed AC power supply was validated with stable voltage output capability in the steady and transient operation, meanwhile the paper also verified that the designed power source influence on a primary current in the transmission line can be ignored.