Abstract: The capillary nozzle with cone angle, which is suitable for the water jet guide laser processing technology, has complex internal flow state. In order to obtain the highly stable water beam in the nozzle, using the method of finite element fluid calculation software simulation and analysis, the flow field in the contracted nozzle which is suitable for the water guide laser technology was simulated and analyzed, and the numerical verification was carried out. The parameter data of generating high-speed and stable water beam were then obtained. The simulation results show that with the increase of the inlet pressure of the plane nozzle, the ratio of the length to diameter of the capillary section decreases, and the fluid enters the nozzle and completely separates from the wall of the capillary section. During this process, no obvious cavitation phenomenon occurs, and the constricted water jet was formed. With the nozzle inlet pressure of 50MPa, the length of the water jet reattachment wall inside the nozzle can reach 90% of the capillary length when the capillary diameter is set to be 0.128mm, 0.07mm, and 0.03mm, respectively. When the tapered tube is 10°, the water beam passing through the tapered section will attach to the wall again. The parameters obtained from the simulation analysis can provide guidance for the selection of the nozzle.