The safety of earth-rock dam relies on whether hydraulic fracturing will happen or not in the core wall. One of the key issues is the mechanism and conditions of hydraulic fracturing. By using the self-made test apparatus, specimens without initial cracks and with 5 different depths of initial cracks are prepared for hydraulic fracturing laboratory tests under 2 different loading rates. Combined with numerical analysis and CT tests, the mechanism of hydraulic fracturing is confirmed as wedge splitting effect. When water pressure is applied onto the “cutting back” formed by initial cracks and the induced force on the “cutting blade” exceeds the critical value, it possibly leads to hydraulic fracturing. Test and analysis results show that the deeper the initial crack on the surface of specimen, and the higher loading rate, the easier hydraulic fracturing occurs. In order to prevent from hydraulic fracturing on earth-rock dams, attentions must be paid on the construction quality and smoothness of upper-surface of the core wall, and the water level of reservoir is recommended for slowly raising.