Abstract: Mechanism of cavitation is introduced. Rationality of cavitating number which is used to judge cavitation and describe cavitating degree of jet is discussed, and it is found that cavitating condition cannot be obtained from cavitating number. The prerequisite for cavitation is that there exists an area in which absolute pressure is lower than or equal to vaporization pressure. To cavitate jet in high pressure environment, special decompression measure must be undertaken. Problems of Rayleigh equations for calculating impulsion pressure under annihilation of cavity to explain the fact that cavitating jet is of greater power to break target object than common jet are analyzed. A new explanation is given that target surface pressure pulse from density changes of cavitating jet which contain cavity is the main factor for breaking target more effectively.