A composite oscillator model is proposed for studying energy dissipation mechanism of atomic-scale wearless friction. The model consists of the whole macroscopic oscillator and the micro oscillators of interfacial atoms. Different influences of the two oscillators on the energy dissipation process of friction are discussed. It is found that the frequency of the interfacial excitation force is the key to energy conversion in the friction process by analyzing the dynamic characteristics of interfacial atoms. In the equilibrium stage, the interfacial force acts integrally and uniformly on each atom because its frequency is nearly zero. In the non-equilibrium stage, however, the distribution of the energy received by the interfacial atoms is not uniform because the frequency of the interfacial acting force is very high. Therefore, the extra energy may be easily transferred to the adjacent atoms to make the energy dissipate. The results show that the composite oscillator model can more clearly explain the energy dissipation mechanism of friction in detail.