Essentially, abrasive waterjet cutting is a process of water erosion. Based on the cutting process of the brittle materials with abrasive particles , the author has studied the cutting mechanism of abrasive water-jet. The author has a discussion on the pressure and its distribution on the subject surface, and the mode and features of water-erosion failure the object is subjected to when it is being cut by the abrasive waterjet, and analyzes the effects on impact energy efficiency of the abrasive waterjet of factors such as de'nsity, velocity and geometric size of abrasive particles and so on.