Hydrajet-fracturing technology depends on a jet stream to achieve a seal for stimulation without mechanical devices. An experiment combined with a numerical simulation method studied the sealing mechanism. It found a low pressure field around the jetting tool and the head of the cavity. Because of the pressure regime, high pressure fluid in annulus will flow to this area and then mix with the jet stream into the fracture. Research showed that three factors influence the sealing pressure. This value will become larger with the jet pressure and nozzle diameter rising. The casing hole plays a seal role to prevent the cavity pressure from disturbing the sealing pressure. Based on the experimental data, a model of sealing pressure is proposed. Microseismic monitoring results prove that treatment fluid is effectively isolated into every fracture for extension. The conclusions and models are significant to further understanding of the hydrajet-fracturing process.