In recent years, the advance of the elliptical resonant cavity and focus cavity is known by many people. There are
homogeneous and multipatternal virtues in the focus dimensional microwave field of the elliptical resonant cavity. It is
very suitable for applying the low power microwave biological effect equipment. However, when designing the elliptical
resonant cavity may meet the problems of complex and huge computation need to be solved. This paper proposed the simple way of approximate processing the Mathieu function. It can greatly simplify the difficulty and decrease the scale of computation. This method can satisfy the requirements of research and development within project permitted precision.