Abstract: Based on the uncoupled fluid-structure interaction theory, a numerical model has been proposed to study the fracture mechanism of rocks under water jet impact. The jet flow characteristics and the stress distributions in the system are calculated with and without taking account of the pore-fluid coupling. The numerical results show if no pore-fluid coupling is considered, the maximum principal stress will be located at the impact surface and the radial distance from the impact center is directly proportional to the corresponding standoff distance, meanwhile the maximum shear stress occurs about half of the nozzle diameter downward the impact center. On the other hand, when water jets impinge on the saturated rocks in which fluid flows according to the Darcys law, the maximum principal stress occurs about 0.4 time of the nozzle diameter downward the impact center. The results can provided some ideas in considering the rock damage criterion for rock failure mechanism study under water jet impact.