Nozzle is the core work component of high pressure water jet, its geometry structure characteristics directly influence the jet quality and work efficiency. The computational fluid dynamics (CFD) method was applied in study for the current widely used cylindrical nozzle flow field inside, with the maximum exit velocity as target variable, then the response surface method was used to optimize the nozzle structure. The study results show that: the cylindrical nozzle axial velocity and pressure present approximate dual characteristic. The nozzle exit diameter, nozzle export cylindrical section length and nozzle contraction angle of nozzle exit velocity has remarkable effect on nozzle exit velocity, and the inlet cylindrical section length and diameter of the inlet has less effect on outlet velocity. Among them, between the nozzle export cylindrical section length and the outlet velocity has approximately a linear relationship, while the export diameter and nozzle contraction angle to the exit velocity has parabolic relationship. The influence nozzle pressure drop factors are nozzle entrance and exit diameter, while the other factors such as nozzle export cylindrical section length, entrance cylindrical section length and contraction angle to the influence of the pressure drop can be ignored.