In view of the problems that the delicate materials often occur cracks at initial piercing moment and during drilling process by high pressure abrasive water jet, carried on the thorough theoretical research. Based on the analysis of high-pressure abrasive water jet operating principle and previous experimental experiences, obtains the water and abrasive effect mechanism on brittle materials at initial piercing moment and drilling process: 1） In the initial phase, when the jet is turned on, at the water jet column and a solid surface contact initial instant, the water jet through the compression waves produces a very high pressure on the solid surface, if the pressure goes beyond the critical pressure that the material could bear, the brittle fracture would happen; 2）In the drilling phase, the compression waves transfer to free surface water jet column, and it takes some time for the abrasive from the tank into the mixing chamber(at most several seconds), in which the water jet acts on the solid material, and produces stagnation pressure, if the pressure is beyond the material critical pressure, the brittle fracture would also happen.