An S-band high-power narrow-band random signal source based on magnetron is developed by taking the magnetron’s advantages, including high power, high efficiency, small size and low cost. Theoretically, the spectrum of a magnetron’s output signal is closely related to its power supply. Herein, the output spectra of magnetron under different cathode currents and anode voltages are experimentally measured. Then, a microcontroller is utilized to control the power supply to generate random variation of anode voltage and cathode current in a specific range. In this way, the output microwave signal of the magnetron would also change randomly in a certain range, thus generating a high-power, narrow-band random signal. Experimental results show that the center frequency, average output power of the narrowband random signal are 2.46 GHz and 1.4 kW, respectively, with DC to microwave conversion efficiency higher than 70%. The high-power narrow-band random signal source developed in this paper has potential applications in electronic countermeasures, communication jamming and anti-jamming situations.