High pressure water jets have strong erosion ability and have achieved excellent results in drilling engineering. Cavitation phenomena occur in the process of jetting, which is accompanied by high temperature and high pressure, and is one of the key factors affecting the erosion ability of jets. The structure of the nozzle has a significant influence on the cavitation ability of the jet. Analysis of the relationship between the flow field characteristics of the cavitation jet and the nozzle structure is an important aspect of the study of high pressure water jets. In this paper, we conducted visualization experiments and we 3D-printed organ-pipe self-resonant cavitating nozzles. The characteristics of the cavitation jet flow field at the nozzle outlet, especially the morphological changes of the cavitation cloud in the flow field were captured with high-speed photography. After image processing, the impact of nozzle structure changes on cavitation generation capacity was analyzed. We used the proper orthogonal decomposition (POD) method to obtain the time-averaged characteristics of the flow field structure. The results show that the resonator is an essential structure that affects cavitation. Increasing the length and diameter of the resonator within a certain range ensure the occurrence of cavitation and the structural stability of the flow field. However excessive size affects the self-resonance of the nozzle and makes it difficult to create resonance. In this paper, the optimal values of nozzle outlet diameter and extension angle are twice the outlet diameter and 40°, respectively. The stability difference of water jets were analyzed by comparing the time-averaged characteristics under different nozzle structures and jet hydraulic parameters. The results show that the resonant cavity in the nozzle structure is the main part affecting the cavitation generation ability. Increasing the length and diameter of the resonant cavity within a certain range is conducive to enhancing the cavitation generation ability of the nozzle and improving the structural stability of the flow field at the nozzle outlet. However, excessive length and diameter of the resonant cavity will affect the self-vibration effect of the nozzle, making it more difficult for the fluid to form resonance when passing through the nozzle, and making the cavitation ability of the nozzle drop sharply. In this paper, the optimal values of nozzle outlet diameter and extension angle are twice the outlet diameter and 40°, respectively. This research provides a better way to study optimization of self-resonant nozzles and cavitating-jet characteristics, which is intuitive and can be a validation for other approaches.