In this investigation, the friction stir processing (FSP) method was adopted to improve the cavitation erosion-corrosion resistance of as-cast Mg-Zn alloy. The microstructure, element distribution, phase identification and microhardness were observed and estimated by using SEM, EDS, XRD and micro-sclerometer. Moreover, cavitation erosion-corrosion resistance was investigated by ultrasonic vibratory apparatus and electrochemical system. The results show that the microstructure becomes finer and more homogeneous. The element distribution also becomes more uniform, and the microhardness is improved by the FSP method. The corrosion product films with a high protective property are easy to form on the surface of the FSP Mg-Zn alloy, resulting in the improvement of corrosion resistance. In addition, owing to the low hardness of the FSP Mg-Zn alloy, the cavitation erosion resistance of the FSP Mg-Zn alloy is not improved in distilled water test. However, FSP samples possess a better cavitation erosion resistance in artificial seawater tests due to their higher corrosion resistance.