<p>This article comprehensively investigates the scouring mechanism of underwater cement paste (UWP) through parameter calibration, flume erosion testing, numerical simulations, and force chain analysis. Building upon the established ARR constitutive model, concurrent calibration involving flowability and rheological parameter experiments confirms that when fluidity ratio, yield stress, viscosity below 3.5, 7.5, 3.5%, the discrete element method (DEM) can effectively simulate fresh UWP performance. A comprehensive analysis of the slurry anti-erosion characteristics was then conducted using a one-sided flow erosion apparatus and 3D image reconstruction technology. Through physical and numerical comparative analysis, the feasibility of simulating anti-erosion performance within an error range of 4.42–10.93% for Ha and Hm values was finally confirmed using the DEM-CFD coupling method. After that, an in-depth analysis of particle force chains and displacement field was further carried out to elucidate the UWP anti-washout mechanism under dynamic water conditions. The results indicate that as the coordination number of cement particles decreases from 5.98 to 4.11 and relative displacement reaches 3.5 times their diameter, cohesive force chains within the suspension dissipate, resulting in the dispersion of the entire cement slurry. The UWP erosion process under dynamic water conditions unfold in two stages, with approximately 74.0% of the force chains rapidly fracturing within the initial 10% scouring time, while the remaining force chains break apart gradually through a slow tearing process.</p>