We developed a new method to apply the continuum model to iterative simulations of an ionic solution. This method is composed of two steps. For the first step, the Poisson-Boltzmann equation is solved to obtain the electrostatic potential. For the second step, the "effective" atomic charge at each atomic center is calculated to approximate the electrostatic potential. This method enables avoidance of solving the Poisson-Boltzmann equation in every conformational change step. The solvation free energy is calculated to within 4% compared to the FDPB method. The calculated conformational preference of the alanine dipeptide is in good agreement with the experimental results. The computational time for the MC simulation of the alanine dipeptide is reduced one-5000th as compared to the result of the simulation with FDPB calculation in every one MC step. The method will be helpful in analyzing the solvent and ion screening effect of proteins and nucleic acids.