<p>This paper uses a high-performance mixed-flow waterjet pump as its research object. Through numerical simulation, we monitor the impeller and the diffuser pressure pulsation of the waterjet pump and analyze pressure pulsation variation patterns of the impeller and diffuser, as well as transient fluid force variation patterns under different cavitation conditions. This is done to reveal the fluid excitation characteristics of the waterjet pump under different cavitation conditions. In the time domain, it was found that the period of the impeller pressure pulsation is associated with the number of diffuser blades. The characteristics of the diffuser pressure pulsation period are associated with the number of impeller blades. In the frequency domain, the impeller and diffuser have different main frequencies and cavitation affects the pulsation spectrum. The impeller surface fluid forces and moments are regulated by rotor-stator interaction, decreasing, and then increasing with the development of the cavitation. The fluctuations are most intense during the fracture cavitation condition, and the phase offset can reflect the flow field instability. Existing studies have mostly focused on the pressure pulsation characteristics of general pump types such as centrifugal pumps, with relatively few studies on marine waterjet pumps, and most of them are limited to single cavitation conditions or local flow field analysis. In addition, existing studies mostly analyze pressure pulsations or fluid forces separately, whereas this study is the first to conduct a coupled analysis of pressure pulsation characteristics and the impeller’s transient fluid forces (forces in X/Y/Z directions, radial force, and three-directional moments). It reveals the law by which cavitation-induced pressure pulsations are transmitted to the impeller surface through interference effects, thereby influencing the fluctuations of fluid forces and moments. This law provides a new perspective for understanding the fluid-structure interaction mechanism under cavitation excitation and fills the broken link in the “pressure pulsation-fluid force-vibration” chain in existing research. This research provides theoretical basis and technical support for the design optimization of waterjet pump and improving its performance and stability, which is of great significance in promoting the development of related fields.</p>