Abstract: In the numerical analysis of axially symmetry electromagnetic field, the finite element method using ring unit with rectangular cross -section has more advantages than other methods. The ring unit characteristic equations in relation to Laplace s, Poissou s and Helmholtz s equa.tions are derived. Using this method, the field structure of cylindrical capacitor and the capacitance and quality factor of the end of laded eoaxial cavity are successfully analyzed.