The charge/discharge, electrochemical impedance, cyclic voltammogram, and cycling behaviors of crystalline K x MnO2·nH2O as cathode material for supercapacitors in Li2SO4, Na2SO4, and K2SO4 electrolytes were compared. The different power and cycling performance of K x MnO2·nH2O during charge/discharge in the three electrolytes were elucidated by analyzing its composition and structure evolution. Compared with the Li2SO4 and Na2SO4 electrolytes, the highest ionic conductivity of K2SO4 electrolyte, the fastest charge-transfer process and slightest structural evolution of K x MnO2·nH2O during charge/discharge in the K2SO4 electrolyte lead to a superior power and cycling behavior for supercapacitor application.