A small amount of cations such as Li+, Na+, K+, or NH4 + is required to stabilize the tunnels during the formation of MnO2. Here, the effect of the content of K ion on crystalline state and capacitance behavior of MnO2 are investigated. It is found that with the content of K ion increasing, manganese dioxide is more likely to maintain amorphous, while adding Li+, Na+, or NH4 + has no such effect. Adding K ion not only delays the conversion of manganese dioxide crystalline, but also delays the phase transformation temperature, which causes the big difference of specific capacitance value from 58 F g−1 for H2/400 to 124 F g−1 for K12/400. Furthermore, the morphology, special surface area, and pore data change dramatically from H2/400 to K12/400. These results have important implications for the characterization of MnO2 in supercapacitor applications.