In this paper, we discuss the generation and evolution of continuous-variable entanglement in a two-mode single-atom laser, where the atomic coherence is induced by two classical microwave fields, which drive the corresponding fine atomic transitions. The results show that the intensity of the microwave field can influence effectively the entanglement properties of the cavity field. In addition, our numerical results also show that the intensity and the period of entanglement between the two cavity modes as well as the total mean photon number of the cavity field can be increased synchronously by adjusting the corresponding frequency detuning.