In the coherent power combing experiment of multi-channel injection-locking of high-power continuous wave magnetron, the analysis on the output character can improve the combing efficiency. An injection-locking system of a 20 kW continuous wave magnetron at S-band is built, which is composed of three small systems: a magnetron signal generating system, an injection locking system and a phase difference detecting system. There are a microwave frequency source and a phase shifter with adjustable amplitude and adjustable frequency in the system. Using the external injection signal, the phase stability, spectrum and phase noise of the output signal of the magnetron are analyzed respectively. The minimum phase difference fluctuation is less than 4°, the maximum is 17°, the locking frequency band varies between 2.9 MHz and 13 MHz, and the phase noise suppression is over 40 dB. The relationships between the injection locking signals and the output signals are also summarized, which lays a foundation for the power combing of magnetrons.