The high-power microwave source does not satisfy the requirements for deep space communications, wireless power transmission systems and high-power superconducting accelerators because of the real-time power control problems. To break through such limitations, an available power control method based on injection locking is proposed. Theoretical analysis and experiment are carried out to prove the performance of the proposed frequency-controlled microwave source based on dual 20 kW S-band high power magnetrons. The experiment results show that the real-time power control combining via the injecting frequency is achieved, when the injection power or free-running frequency of magnetron is unequal. Simultaneously, the system shows an output power control range of 3.0 dB, a nearly 4.0 MHz injection-locking bandwidth, and a sharp spectrum with an excellent spur suppression ratio of -65.0 dBc@500 kHz. The highest combining power output of 33.9 kW with the system efficiency of 86.6% is also proved. Such researches play an important role in the development of high-power microwave applications.