Abstract：Three jet stages in the flow structure and the rock fragmentation mechanism of strike，impact，water wedge，cavitations erosion，cutting of water jet，and the course of rock fragmentation，are analyzed. The frontier problems of rock fragmentation by water jet at present are studied，where it is suggested to study the course of rock fragmentation by water jet from the angle of the coupling effects of water jet and rock and effects of the dynamic fragmentation of rock. A new scheme of drilling a hole by a high pressure water jet in the coal seam is put forward. A continuous steel pipe twisting on a roller connects the water jet bit with the high pressure water pump，and a continuous drilling job in the coal seam can be performed，which is of drilling automatism and can quicken the drilling. The nonlinear relation between the pressures and the distances of water jet through the test datum and the delivery samples of the drilling slag are analyzed，and it shows that the slag particle by the water jet drilling is larger than that by the ordinary drilling. The conclusions are drawn that it has a critical pressure of rock fragmentation by water jet，and that the pressure and the flux of water jet have nonlinear correlative laws with the efficiency of rock fragmentation. Compared with the flow rate，the water jet pressure plays a more important role on the rock fragmentation.
