Future moderate-band (MB) high-power microwave sources will take advantage of having a small size and a low cost. However, the output power of existing designs is limited by the dc voltage supply. To tackle this issue, this article proposes a novel scalable high-power microwave source based on the concept of coherent stimulated emission. The proposed microwave source consists of an antenna array, in which the two arms of each dipole antenna are connected with a fast-rise time switch and charged by a dc voltage source. Each antenna unit is triggered by the incoming wave radiated from the previous unit and then oscillates and correspondingly produces a new electromagnetic wave to trigger the next one. Both simulations and experiments show that the output waves of multiple antenna units are in phase, and the output power is accordingly enhanced. Compared with the existing MB high-power microwave sources, the proposed microwave source is able to significantly boost the maximum output power by applying more automatically triggered antenna units, featuring strong scalability and less limitation by the dc voltage supply.