In this paper, the phenomenon of the asymmetric energy transmission is numerically investigated in the forbidden band of the electrical transmission line formed by two nonlinear segments which are identical in structure but different in inductor parameter. By considering the driving voltage at the frequency within the forbidden bands of both segments, the carrier of the asymmetric energy flux is the nonlinear wave beyond the band, instead of the linear wave in the passband, and the mechanism is closly related to the nonlinear supratransmission. To further understand this phenomenon in electrical transmission line, we also study the correlations between the energy intensity and the circuit parameters. Finally, we investigate the dependency of the voltage threshold on the driving frequency in physical experiment, and the result is qualitatively identical with that calculated by using equation.