Abstract: This paper applies the theory of kinematics and space charge wave to conclude an experiential formula for calculating the phase of the modulated current at the entrance of the gap of the middle cavity. It also applies a nonlinear theory of cavity excitation by modulated electron beam to calculate the amplitude and the phase of the gap voltage of the middle cavity and the output cavity, and presents an experimential formula for calculating the phase of the modulated current at the entrance of the gap of the output cavity. With these theories and 2D PIC, it estimates the phase of the modulated current at the entrance of the gap of the middle cavity and the output cavity as well as the amplitude and the phase of gap voltage in the middle cavity and the output cavity. The errors of the phase of the modulated current at the entrance of the gap of the middle cavity and the output cavity are 2.627° (model 1) and 3.857° (model 2); the relativistic errors of the amplitude of gap voltage in the middle cavity and the output cavity are 1.47% and 5.42%, the error of the phase of gap voltage in the middle cavity are 4.017° (model 2) and 5.427° (model 3), and the error of the phase of gap voltage in the output cavity is 12.32°. Finally, the paper analyzes the phase of the modulated current versus the propagation distance in three models by 2D PIC simulation.