<p>High molecular weight polymers (HMWPs) exhibit exceptional adsorption properties, enabling them to bind cement particles into a cohesive mass, thereby significantly enhancing the washout resistance of concrete mixtures. This study investigates the mechanism of polymer adsorption on the washout resistance of ultra-high-performance concrete (UHPC) using polymers at dosages of 0%, 1.5%, 2.5%, and 3%. Techniques such as mechanical strength testing, mercury intrusion porosimetry (MIP), water absorption analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS) were employed to explore the underlying mechanisms. The results indicate that while HMWPs do not significantly alter cement hydration, they absorb water from the surrounding slurry, reducing the flowability of UHPC by 16.8%, 24%, and 33.7% at polymer dosages of 1.5%, 2.5%, and 3%, respectively. Porosity increased from 15.87% to 16.92%, 17.34%, and 18.42%, leading to compressive strength reductions of 4.3%, 5.3%, and 20.1%, respectively. Despite these changes, HMWPs effectively adsorb cement particles, enhancing underwater washout resistance with mass losses reduced to 0.0%, 18.6%, 7.7%, and 0.2% at the respective dosages. These findings underscore the potential of HMWP in optimizing underwater UHPC applications.</p>