Research of interactions between high-power microwave and plasma is of importance for both microwave discharge and electromagnetic compatibility. Based on wave equations, fluid equations, and Boltzmann equations of plasma interactively, a theoretical model of interactions between high-power microwave and plasma was established. Combined with characteristic parameters of plasma, the model was solved numerically by the method of finite difference in time domain to analyze variations of plasma electron density and transmission property of high-power microwave. Results show that heating of electrons by incident high-power microwave makes nonlinear effects become obvious in the plasma, and gas breakdown increases plasma electron density which results in the enhancement of reflection of microwave and the of decrease transitivity. The model mentioned above and results related are instructive for protecting against high-power microwave and electromagnetic pulse.