Abstract
Fom the view of pure math, to study an open resonator is a problem of solving Maxwell equation with boundary condition. From the view of pure physics, we can understand and learn the property of wave by studying this problem. And we can also have deeper view about the phenomenon of resonation. In fact,there are many kinds of resonation and they have some common properties. The most important character can be analyzed by using spectrum analysis. And we have many discussions about this technique. In applied physics, we can build the supplement of energy of electron beams in SSCR. On the other hand,we can also study the research of gyrotron by knowing property of an open end reasonator.