This paper introduces a new equivalent parallel series circuit of the single-feed circular polarization microstrip patch antenna based on Cavity Model Theory and gives the calculating formulas of axial ratio and reflection coefficient. The relationship between axial ratio and reflection coefficient of the single-feed circular polarization microstrip patch antenna is also analyzed in the paper. So, the axis ratio can be estimated according to reflection coefficient calculated by the formula put forward in this paper. The comparison between this result and that calculated with circuit parameters validates?the correctness of the method.