Abstract: The oscillating characteristics of self-excited waterjet determine the jet's resonating and erosion effects, which are greatly affected by nozzle outlet geometry, but the influence law is unclear. In this study, firstly, the influence factor of the jet's oscillating characteristics is analyzed based on the self-excited waterjet's mechanism, and the internal relationship between nozzle outlet geometry and oscillating characteristics is studied theoretically. The ambient pressure is adjusted to make the self-excited frequency changing continuous, and the self-excited waterjet's pressure and noise signal are captured based on the signal detection method. Meanwhile, time-frequency analysis is carried out to decouple the self-excited frequency from the acoustic natural frequency. Experimental results indicate that the nozzle outlet geometry, including the expansion angle and length, could significantly affect on the self-excited frequency and change the oscillating characteristics. The nozzle expansion angle determines whether the self-excited oscillation occurs or not. An appropriate expansion angle can promote the vortex ring shedding, and then cause the initial pressure excitation. However, an effective self-excited oscillation cannot occur when the expansion angle is 0°. When the expansion angle increases from 10° to 40°, the self-excitation frequency decreases first and then increases. The expansion angle also can promote oscillation initiation, and the incipient cavitation number increases with the decrease of the expansion angle in a specific range. The expansion length will affect the self-excited frequency and the cavitation number of initial oscillation. In the present working condition, the self-excited frequency decreases, and the incipient cavitation number increases with the expansion length. The effects of nozzle outlet geometry on the oscillating characteristics of self-excited waterjet are uncovered, and the mechanism is preliminarily explored. In the meantime, the influence of nozzle outlet geometry on the incipient cavitation number is qualitatively discussed, which supports a further understanding of the oscillating characteristics of self-excited waterjet.