  We present a mean-photon-number dependent variational method, which works
well in whole coupling regime if the photon energy is dominant over the
spin-flipping, to evaluate the properties of the Rabi model for both the ground
state and the excited states. For the ground state, it is shown that the
previous approximate methods, the generalized rotating-wave approximation (only
working well in the strong coupling limit) and the generalized variational
method (only working well in the weak coupling limit), can be recovered in the
corresponding coupling limits. The key point of our method is to tailor the
merits of these two existing methods by introducing a mean-photon-number
dependent variational parameter. For the excited states,our method yields
considerable improvements over the generalized rotating-wave approximation. The
variational method proposed could be readily applied to the more complex
models, for which an analytic formula is difficult to be formulated.
