The Berry phase of the Rabi model beyond the rotating-wave approximation is investigated. By use of two unitary transformations, the analytical expressions of the Berry phase are obtained approximately for the Rabi model. The obtained analytical solutions reproduce accurately the high-accuracy numerical results of the Berry phase in the strong coupling regime. It is found that the Berry phase of the ground state is relevant to the second-order terms of the coupling strength, and the Berry phases of the first and second excited states are mainly relevant to the first-, second- and third-order t...
