Open cavities are commonly used in research of gyrotrons . The lack of well-defined geometrical boundary cause the distinctive features that is basically different from close cavity. Hence, we understand its physical peoperty and phenomenon by numerical simulation . We introduce the time domain models and declare the numerical algorithm and the boundary condition. The relation of quality factor and resonant frequency in time domain models is explained. We also discuss the power flow in open cavities . By changing the shapes and materials of open cavities , we analyze the flow and decay of energies. We derive the formula of power flow by electrodynamics and check our guess by numerical simulation. Finally we try to give an appropriate physical explanation for the results.