Based on the composite oscillator model of atomic-scale wearless friction, a quantum mechanics model is proposed for analyzing the energy dissipation. The analysis indicates that the energy dissipation is discontinuous in a sliding process. The energy absorbing ability of an atom on a contact surface is discrete when the atom jumps from one equilibrium state to another. At the same time, the ability of a contact surface atom to absorbe the interfacial potential energy is stronger in a high energy state than in a low energy state. This will provide the theoretical basis for analyzing the periodical change in friction.