Abstract: High cross-linked polyethylene (HXLPE) acetabular cups and ceramic joint heads are commonly used in artificial joints to form friction pairs. In order to improve the inter-surface lubrication performance, the grafted poly(2-methacryloyloxyethylphosphorylcholine) zwitterionic polymer brush (PMPC) was prepared on HXLPE surface by UV-induced grafting method, the PMPC solution was used as a lubricant to make the friction interface had free PMPC to study the effect of grafted-free state PMPC synergy on the lubricating properties of friction pairs. In order to further simulate the effect of metal ions in human tissue fluid, studied the influence of Na⁺, Ca²⁺ and Al³⁺ on the lubrication characteristics of PMPC between the surfaces of artificial joint friction pairs. The results showed that under the load of 3 N, when only the grafted PMPC was used to lubricate the surfaces of the artificial joint friction pairs, the friction coefficient was 0.158 and the fluctuation was frequent. When the grafted-free state PMPC acted synergistically on the surface of the friction pair, the friction coefficient was reduced to 0.072 and remained stable within 120 min of the experiment. Under the 2 N load, the friction coefficient of the synergistic effect of grafted-free state PMPC was 0.056, when low-valent metal ions Na⁺ and Ca²⁺ were added to the lubricant, the friction coefficient increased slightly, however, when the high-valent metal ion Al³⁺ was added, the attraction between the molecules of PMPC increased, the original hydration layer uniformity was destroyed, so that the friction coefficient increased to 0.118, which was twice that of the synergistic effect of grafted- free state PMPC without interference of metal ions.