  We present a generalized variational method to analytically obtain the
ground-state properties of the unsolvable Jaynes-Cummings model with the
ultrastrong coupling. An explicit expression for the ground-state energy, which
agrees well with the numerical simulation in a wide range of the experimental
parameters, is given. In particular, the introduced method can successfully
solve this Jaynes-Cummings model with the positive detuning (the atomic
resonant level is larger than the photon frequency), which can not be treated
in the adiabatical approximation and the generalized rotating-wave
approximation. Finally, we also demonstrate analytically how to control the
mean photon number by means of the current experimental parameters including
the photon frequency, the coupling strength, and especially the atomic resonant
level.
