Pyramidal neurons in the electrosensory lateral line lobe (ELL) of weakly electric fish activate in an environment of time-varying electric fields, which are generated by the fish itself, while how these pyramidal neurons would behave or what kinds of firing patterns these neurons would produce under different electric fields is still unclear. In this research, the firing behaviors of ELL pyramidal neuron under DC and AC electric field stimulus are investigated in a two-compartment neuron model. By means of numerical simulations we show that firing patterns of the model ELL pyramidal neuron are much diverse under different values of DC electric field, and neuronal spike frequency exhibits a monotone decreasing trend with the linearly increased DC fields, moreover, the transition mode between these firing patterns with the variation of DC electric fields demonstrates an explicit periodic route. While for AC electric fields, neuronal firing frequency periodically transforms with the increase of AC frequency, particularly, a special transition pattern (from multi-period bursting to spiking) repeatedly appears with the change of AC frequency. Our simulation results indicate that ELL pyramidal neurons fire dynamically under the time-varying electric fields, the diversity of firing patterns and their periodic transition modes may imply the potential roles of these dynamical firings in the coding strategy of sensory information processing.