Abstract: Ultra-high pressure water technology is widely used in petroleum engineering.At present,ultra-high pressure jet,assisted by mechanical breaking of rock,is considered as the most potential and feasible approach to improving drilling speed.In this paper,the law of rock drilling with ultra high pressure water jet under submerged conditions is studied based on both lab and field experiment.The main factors and relations between these factors that determine the rock breaking effect are discussed as well.This provides a basis for further study on ultra-high pressure jet assisted by mechanical breaking of rock and on PDC bit with ultra-high pressure.The result shows that the main factors affecting the breaking of rock are jet pressure,the standoff distance,the nozzle moving speed,the jet angle.The higher the water jet is,the better the rock breaking effect is.And the optimum standoff distance increases with the rise in pressure.The optimum standoff distance increases to 32.5 times larger than the nozzle diameter when the pressure is 200 MPa.Under laboratory conditions,the highest efficiency of rock breaking is obtained when the pressure equals to 150 MPa,and the optimum jet angle is 14 degrees.According to the experimental results,the configuration of cutters and nozzles is optimized and special equipment and tools are made.Great success is achieved in the field experiment.