There are several disadvantages in the current electromagnetic theory of reverberation chamber analysis. The first disadvantage is that the current theory can only estimate number of electromagnetic modes and statistic uniform field distribution under high order modes circumstance inside a reverberation chamber, not give a systematic electromagnetic analysis of operating reverberation chamber. The second disadvantage is the current theory has more experiential analysis and less theoretical calculation, suitable for high order modes reverberation chamber but not applicable to the case of low order modes. In this paper, a method of superposition of eigenfunctions is used to derive formula of electromagnetic field distribution in an active operating reverberation chamber, and to present an analysis and systematical categorization of various EMC operating states