Abstract: Abnormal gushing at two drainage holes of Jiangkou hydropower station occurred on June 20, 2007. One was the drainage hole between the grouting gallery at left bank and hoist shaft on the elevation 232 m, while another was at the junction part of the second dam gallery and the drainage gallery of right bank on the elevation 170 m. As heavy rain on June 19~20, abnormal gushing with mud from drainage holes was found in the beginning. Then, mud and sand in the gushing water decreased and returned to normal after 30 hours. By comprehensive analysis, the cause reason was concluded that the engineering treatment before changed the seepage path of underground water, as a result, the filling material in the cave was rushed out because of serious corrosion. The treatment measures included the formation of the impervious curtain, the replacement of III-404 sandwich, the closure of KW11 and K8~K12 cave systems and the existence of F35 fault which blocked or changed the pathway of the original groundwater seepage. Accordingly, treatment advices were presented.