Percussion pulse jet has powerful capabilities on hard metical fragmentation, such as rock, concrete and ceramics. However, its complicate generation process make the design works on this become hard. This is the reason why the development of this technique has been restricted for a long time. In presented works, a simplified finite difference model combined with multi-collision equations of pistons was put forward. The main hydrodynamic parameters and the motion of the pistons were numerically calculated. A hammer-driven percussion jet generator was designed and implemented for the tests under same operating conditions with modeling works. By comparing the pressure data and the numerical results, those were demonstrated that the mode was reasonable and the neglects were acceptable. Conducting numerical experiments using this model, valuable reference information can be achieved for percussion pulse jet design.