Based on the open trapezoidal three-energy level atomic model and density matrix equation theory, the variation of population with time during double-color and double-resonant enhanced multiphoton ionization is numerically simulated under different parameters. When the frequency detuning of two lasers is zero, the populations of the ground as well as first- and second-excited states exhibit damping Rabi oscillation with time. The oscillation frequency of the population distributed in the first-excited state is twice that in the ground and second-excited states. Large population inversion occurs between the second-excited and ground states and exhibits the possibility for the output of coherent light with short wavelengths. Furthermore, the oscillation frequency and population inversion increase with Rabi frequency. Population inversion is evident as the two lasers become synchronous.