The motion of electrons is affected by microwave in RF cavity. Meanwhile electrons can also generate radiation and have influence on microwave in the cavity. Ac cording to Maxwell theory, RF field in the RF cavity can be expressed by modes. So equations about such actions are established according to Maxwell theory and Lorentz force acting on electrons in the cavity. These equations are also called self-consistent equations for action between microwave and electron beam in RF cavity. Based on the equations, the single-gap RF cavity is investigated one- dimensionally (including how microwave increases and saturates in the cavity) an d the factors influencing the output power are also studied. It is concluded tha t nonlinear action is very important, and that output power is not only determin ed by cavity structure but also by the beam modulation.