The force model of hydro-piercing was established, and the influences of the gap between punch and die and the change of liquid pressure in pipe on the force state of deformation region were analyzed. Then, the stress distribution characteristics in the process of hydro-piercing were expounded. Combined with the application of high strength steel hydroforming part, the defects and failure forms of high strength steel hydro-piercing were pointed out, and the technical difficulties were put forward. The results show that the gap between punch and die directly impacts on the quality of hydro-piercing, and reducing the gap can change the stress state and improve the quality of punching. When the liquid pressure in pipe is low, the stress state of material in the deformation zone changes from two-way compression and one-way tension to two-way tension and one-way compression with the punch moving forward. However, when the liquid pressure is high, the material in the deformation zone is always in three-way compressive stress state, so the quality of piercing can be improved by raising the liquid pressure in pipe. For high strength steel parts, it is necessary to optimize the punch materials and select appropriate heat treatment methods in order to improve the service life of punch.