Molecular dynamics method is the most powerful numerical technology in molecular simulation in the scale of nanometer, but it is time-consumed and hard to be used to multiscale simulation not only because the large number of atoms in a complex molecular system but also the femtosecond order of time step requirement. To enable a larger time step and less time-consuming in molecular simulation, a freezing atom method (FAM) is developed by using of the principle of movement discomposing and composing of particles. It is shown that in case of weak coupling between high- and low-frequency process, the use of FAM enable a time step 10(5) longer than the classic molecular dynamics method and higher accuracy of convergence.