Ultrasonic shot peening is a pre-stressing process that enhances the life span of mechanical parts. The spheres constituting the shot are set in motion with a sonotrode and impact the treated part to generate a compressive residual stress field, hence increasing fatigue life. A model predicting the dynamics of the shot within the ultrasonic shot peening chamber is proposed. The influence of the initial conditions on the shot dynamics is first studied. The model is then used to obtain predictive values for the impact density and the spatial distribution of the velocity of the spheres before the impacts. The influence of the ultrasonic shot peening parameters is also evaluated. The variation of the geometry of the shot peening chamber is further considered with the use of a prismatic geometry.