The EM field of electron beam can be efficiently extracted by open cavities. Therefore, open cavities are commonly used in gyrotrons and gyroklystron. Often, we don’t have the analytic solution of it. We dive deep to interpret its physical property by numerical simulation. This article begins with basis theorem and boundary conditions. Then, we will go through the algorithm used in our numerical computation. We would like to investigate the effect caused by the mismatched load attached to the open cavity. So we can learn more about the property of open cavities with mismatched load.