We use the two-oscillator model to describe the coupling between the RLC loops. However，the two -oscillator model is described by two second-order differential equations，the solving process is complicated. Therefore， for RLC coupling loop，we study theoretically and experimentally the correlation between the coupled-mode theory ( CMT) of the first-order differential equations and the two-oscillator model of second-order equations. The result shows the CMT can generalize the electric circuit system.