The motion equations of particles were reduced to the generalized Duffing equation with a soft spring characteristics in the framework of classical mechanics and dipole approximation by using Moriere potential. The motion behavior of the system in the vicinity of the main resonance was analyzed by the average method, and also the stability and relaxation behavior in the system were discussed, and the critical parameters of the system was derived. The results show that the stability of the system relates to its parameters, only by appropriately adjusting these parameters, the stability can be guaranteed in principle.