The rapid development of marine concrete structures and the sharp shortage of freshwater resources contribute to the wide investigation of seawater-mixed ultra-high-performance concrete (swuhpc). However, few studies have investigated the chloride ions (cl-) binding mechanism of swuhpc. Herein, the chloride binding experiments and molecular dynamics (md) simulation were carried out to reveal the physically and chemically bound cl- mechanisms of swuhpc. The results of the experiments clearly demonstrate that the addition of silica fume (sf) led to a significant decrease in the capacity of cl- binding. Conversely, the incorporation of metakaolin (mk) resulted in a marked increase in the content of chemically bound cl-. Furthermore, it is revealed through md simulations that the amount of physically bound cl- heavily depends on the ca/si ratio of c-s-h. A higher ca/si ratio results in a stronger electrostatic effect of the c-s-h surface on cl-, which increases the physical binding of cl- via ca-cl bonds. In addition, it is found that al[6] and ca in the interlayer region of c-a-s-h formed the main structure layer (ca4al2(oh)122+) of friedel's salt, and then chemically adsorbed cl- in the pore solution. These findings provide novel nanoscale insights regarding the physically and chemically bound cl- mechanisms of swuhpc.