A theoretical model is proposed to illustrate the periodic dissociation process of NaI molecule. The kinetic energy release (KER) of the fragments is obtained from the theoretical analysis and the quantum wave packet simulation, respectively. It is found that the theoretical model can predict the periodic dissociation results with four parameters, which are expansion coefficient of the eigenstates cE, attenuation factor of the dissociating wave packets √α, the vibrational period of the excited state τe and the time delay τ between the first two dissociation wave packets. The attenuation factor √α is equal to 1 due to the different photofragments corresponding to the same probability amplitude. Furthermore, the inherent nonadiabatic coupling only causes the population transfer from the excited state to the ground state, without influencing the periodic motion of the wave packet.