Abstract: The mechanism of hydraulic fracturing of the core of earth core dams was investigated,and it was considered that the potential permeable weak links and wedging action of water pressure due to rapid reservoir filling were the important conditions of the propagation of hydraulic fracturing in the dam core.A new type of apparatus was developed for testing hydraulic fracturing caused by wedging action due to the pressure of water entering the weak links of soils.A series of tests were performed to study the wedging action of water pressure in the permeable weak links and the induced action for hydraulic fracturing with the mixed core materials in Nuozhadu High Earth Core Dam.The test results verified the reasonableness of the aforementioned considerations.