  We develop an efficient numerical approach for simulating the high-power
dispersive readout in circuit quantum electrodynamics. In the numerical
simulations of the high-power readout, a large-amplitude coherent state induced
in a cavity is an obstacle because many Fock states are required to describe
such a state. We remove the large-amplitude coherent state from the numerical
simulations by simulating the dynamics in a frame where the amplitude of the
coherent state is almost absent. Using the developed method, we numerically
simulate the high-power dispersive readout of the two-level system and the
transmon. Our proposed method succeeds in producing reasonable behaviors of the
high-power dispersive readout which can be deduced from the photon-number
dependence of the cavity frequency: The high-power dispersive readout works in
the two-level-system case while it does not work in the transmon case.
