Graphite is the most main anode material for lithium-ion batteries (LIBs) and potassium-ion batteries (PIBs). However, the graphite anode in PIBs using traditional dilute ester-based electrolyte systems shows remarkable capacity fading, which is in contrast with extraordinary cyclic stability in LIBs. More interestingly, the graphite anode in highly concentrated electrolyte for PIBs exhibits outstanding cyclic stability. Unfortunately, this significant difference in cycling performance has not raised concern by now. In this work, for the first time, by comparing cyclic stability and graphitization degree of graphite anode upon cycling, we reveal that the underlying cause of the capacity fading of graphite anode in PIBs is not the larger volume change of graphite caused by the intercalation/de-intercalation of potassium ions, but the continual accumulation of solid electrolyte interphase (SEI) on graphite surface. By X-ray photoelectron and nuclear magnetic resonance spectroscopies combined with chemical synthesis, it is concluded that the accumulation of SEI may mainly come from the continual deposition of a kind of oligomer component, which blocks the intercalation/de-intercalation of potassium ions in graphite anodes. This finding clarifies crucial factor determining the cycling performance of graphite anode and provides a scientific guidance for practical application of graphite anode in PIBs.