Abstract: The effects of turbidity, organic substance and inorganic ions on inactivation of Cryptosporidium with the ultrasonic frequency of 19.8kHz,power of 151W and temperature of (20±1)℃ were investigated. Additionally, the mechanism of ultrasonic inactivation of Cryptosporidium was investigated by scanning the electron microscopy and dynamic analysis. The results indicated that the inactivation rate increased at first and then decreased in the range of 0-21.46NTU. During this process, the highest inactivation rate was 94.7%, when the turbidity was 1.13NTU; the inactivation rate increased with the increase of the concentration of humic acid; the inactivation rate reduced to 78.4% when the concentration of humic acid was 50.0mg/L. The effects of Cu²⁺, Mn²⁺and SO₄^2- on the inactivation was negligible, since the inactivation rate merely heighten 0.7% compared Cu²⁺ concentration of 0.5and 0mg/L; The inactivation rate dropped with the increase of CO₃^2- concentration. It reduced to 82.5% when CO₃^2- concentration was 150mg/L. Moreover, the morphological observation by scanning electron microscopy showed that ultrasound was able to damage the cell structure of Cryptosporidium. It was suggested that the mechanical shear action produced by cavitation was the main mechanism of inactivating the species via ultrasound due to the free radical oxidation, which contributed to inactivation was 15.6% through kinetic constants analysis.