Injection phase-locking is the basis of microwave coherent power combining between magnetrons. The experiments of injection locking on a 15 kW large power S-band magnetron and the relation between the injection power and locking bandwidth are presented．The experimental results show that the 15 kW S-band magnetron achieves the phase-locking successfully. The locking bandwidths under various injection powers are studied .The experimental results show that the locking bandwidth becomes wider with the increase of injection power. The locking bandwidth reaches 5 MHz while the injection power is 165 W.It shows that the 15 kW magnetron is a powerful candidate for large power coherent power combining. The study lays the foundation of power combining between multiple large power continuous wave magnetrons.