Water jet is a tool for cutting water through a column of water that is ejected by high pressure. The water knife is characterized by no thermal stress, so it can be cut without chemical change or deformation in the process of cutting without high temperature. The water knife is divided into two types of ultra-high pressure and low pressure according to the output pressure. The pressure of ultra high pressure is 250-600Mpa for the work. It is often applied to the mechanical machining of metal industry. The other type of low pressure type is 25~50Mp for its working demand pressure. This type of water knife is used in situations where no light and high temperature can be used, so it is also known as "Safe water knife".