The tribological behaviour of poly(sodium 4-styrenesulphonate) (PSS) as additive in the base fluid of waterglycol hydraulic fluid was evaluated with four-ball machine and Optimol SRV-IV (Optimol Instruments, Munich, Germany) oscillating friction and wear tester, and its viscosity-enhancing performance was also investigated. The results show that PSS exhibits excellent friction-reducing and anti-wear properties in the base fluid and can significantly improve the latter's welding load and viscosity. The worn surfaces were characterised by scanning electron microscope and X-ray photoelectron spectroscopy. The analytical results of typical elements on the worn surfaces demonstrate that PSS molecules adsorb on the rubbing surface and form a stable chemically absorbed film through the interactions between the sulphonate anions and surface metallic atoms during the sliding process, which contributed to improve the tribological properties of the base fluid.