The mechanisms and mechanical processes of water bursts caused by tensile-shear failure and compression-shear failure of fractures in karst tunnel construction are analyzed according to the hydraulic fracturing principle. Results show that the critical hydraulic pressure of water burst is affected by trend and location of fractures and other factors. Through analyzing the distribution characteristics and gradient of water pressure in the process of fracture propagation, the critical water pressure of tensile-shear failure is revealed greater than that of compression-shear failure. And the crack extension is discontinuous under the action of hydraulic fracturing. The research results are of significance for the prevention and treatment of water burst in the process of tunnel construction.