To investigate the distribution and characteristics of slamming loads on a self-propelled jack-up wind turbine installation vessel (WTIV) during transit and preloading conditions, a three-dimensional hydrodynamic numerical tank model was established based on Computational Fluid Dynamics (CFD). The generation mechanism, time-history curves, and spatial distribution of slamming loads under operational sea states and two typical air gap conditions were obtained and analyzed. The results show that under transit (free-sailing) conditions, the peak slamming load increases with wave height and wave steepness, primarily depending on the water-entry velocity at the slamming point and the vessel's hull flare. During preloading operations, a trapped air cushion beneath the hull is significantly compressed and struggles to escape, leading to higher slamming loads at smaller air gaps. In the same region, slamming loads under beam seas are 6% to 14% higher than those under head seas, while also inducing a lateral horizontal load of approximately 300 t. The characteristics of slamming pressure obtained in this study provide critical references for the structural safety of similar vessels and safe operations during preloading conditions.