Abstract: Balancing mechanism of a flying capacitor is the key to normal operation and capacitor voltage control in Buck/Boost converters with input and output sharing the ground. In fact, the flying capacitor has a certain degree of self-balance ability. However, affected by many factors, it is hard to be used to replace the active control of flying capacitor voltage. The deep analysis on how the different factors influence the self-balance ability has not been provided in publications. Hence, based on the Buck/Boost converters with input and output sharing the ground, this paper establishes the harmonics model. How the error of drive signals, AC impedance of converter loop, bus capacitor, load resistance and flying capacitor influence the self-balance ability is analyzed quantificationally based on the model. Then, it can be found that AC impedance of the converter loop is the key that contributes to the self-balance ability of the flying capacitor. Finally, the analyses are verified by the experimental results.