An electron floating on liquid helium is proposed to be trapped (by a microelectrode set below the liquid helium) in a high-finesse terahertz (THz) cavity. The two lowest levels of the vertical motion of the electron act as a two-level "atom," which could resonantly interact with the THz cavity, and thus the famous Jaynes-Cummings model (JCM) and driven JCM could be implemented. The numerical results show that, for the typical parameters of the cavity and electrons on the surface of liquid helium, strong coupling between the artificial atom and the THz cavity could be obtained.