Substantially, the process of biological treatment for wastewater is the process of bacterial energy metabolism. In this paper, from the viewpoint of energy and based on the conservation of mass and energy, an energy metabolism equation is constructed. Through the experimentation, the parameters in equation are measured. Through this equation the reason for SRT choice of activated sludge system is explained from the viewpoint of microcosmic. This equation is adaptable to optimizing the SRT of activated sludge system in the condition of low-organic load thereby to optimizing the energy distribution of activated sludge system for energy saving