Power Balance(PWB) method is utilized to quickly estimate the magnitude of the mean electric field in an electrically large cavity with rectangular aperture when it is illuminated by a plane electromagnetic wave. As the incident frequency is increased, results obtained by PWB method are more consistent with the results of full-wave analysis software. In order to get more complete information of EM-field environment in the cavity，Probability Distribution Functions(PDFs) of the normalized electric field in the cavity are generated at different incident frequencies. It is found that at each specific frequency when conductivity of the cavity wall gets higher from 10 S/m to 107 S/m，the PDFs get flatter and tend to be stable；and when the conductivity of the cavity wall is 106 S/m，the PDF of the normalized electric field is similar with the one obtained in the case of 107 S/m. The reason is explained taking the incident wave frequency of 10 GHz as an example according to PWB.