Abstract: Cryptomelane-type hydrous manganese dioxide (CRYMO) was prepared by using manganese sulfate and potassium permanganate as raw materials. Potassium ion-sieve（CRYMO-H）with a crystal structure of α-MnO2 was obtained by acid-modification of CRYMO. CRYMO-H showed a high saturated ion-exchange capacity for K+ as measured as 4.6429mmol (181.54mg) K+/g ion-sieve in 0.1mol/L K+ solution. The exchange isothermal curves of H+-K+ on CRYMO-H were measured at 15℃，25℃，35℃，45℃ respectively. The activity coefficients were calculated by using Pitzer electrolyte solution theory，other thermodynamic constant such as equilibrium constant ， ， and during the exchange process were obtained. The equilibrium constant reduced with increasing temperature，the selectivity of CRYMO for K+ was higher than the original ion (H+)，the exchange process was exothermic and spontaneous( <0).