Abstract: On the base of the internal energy flow of the planetary hybrid system, a theoretical fuel consumption model, a fuel saving model, and a fuel-saving rate model were established to meet the requirement of the fuel saving factor analysis. According to the characteristics of super-capacitor and planetary hybrid system, the coupling relationship among the average fuel consumption rate, the efficiency of each motor and the mechanical efficiency was explored. The quantitative analysis on the direct or indirect effects of the factors on the fuel-saving rate and the principles were presented by decoupling each factor. Results of these analyses clarify the fundamental approaches to saving fuel in the planetary hybrid system and the contribution of each factor to fuel economy, which shows the key points and directions for the depth-saving of hybrid system, and provides great guiding significance for improving the efficiency of the economic optimization of hybrid vehicles.