Abstract: On the foundation of Adler classical phase-locking condition, parameter area of injection ratio and frequency mismatch about injection phase-locking of a high-power relativistic magnetron has been studied by particle simulation under the consideration of the highly nonlinear beam-wave interaction in the interaction space. The influences of injection ratio and frequency mismatch on phase-locking condition have been analyzed respectively. For the injection phase-locking of a relativistic magnetron with 2.4667GHz resonance frequency, the frequency mismatch 35MHz as the injection ratio 0.25, and the injection 0.15 as frequency mismatch 20MHz both realize stable phase-locking. Meanwhile, a nonlinear parameter, lying on the level of nonlinear interaction, has been introduced to modify the Adler classical phase-locking condition. This parameter in the present model is about 7.Larger region for phase-locking can be predicted by this modified theory.