In order to explore the influence of cavitation on the stability of the micro-water beam of the water guide laser, the numerical simulation analysis of the water jet from a typical nozzle was carried out by using the computational fluid dynamics method without considering cavitation and considering cavitation respectively, and the generation mechanism of cavitation and its influence on the stability of the micro-water beam was studied. The calculation results show that under the nozzle structure with the inlet diameter of 0.1 mm, the length-diameter ratio of the cylinder section of 1, and the sharp upper edge, with the inlet pressure increasing from 0.2 MPa to 16 MPa, the re-attachment length of the water jet after separation from the nozzle wall gradually increases, and cavitation will lead to the extension of the length. When the inlet pressure is less than 6 MPa, the water jet, without considering cavitation, will flow down the nozzle wall after stabilization, and the addition of a cavitation model can make the water beam form a flipped flow. The simulation results have important reference values for the design of nozzle structures that meet the requirements of water-guide laser technology.