A potentially good extreme‐pressure (EP) additive, diisobutyl polysulphide (PS‐i‐Bu), the main component of which is diisobutyl trisulphide, was synthesised at low temperature and under atmospheric pressure. Its load‐carrying capacity (PB and PD values) and antiwear and antifriction performance as an additive in a diester synthetic lubricant were investigated using four‐ball test machines, and compared with those for traditional sulphurised olefin (SO). Results show that the novel PS‐i‐Bu as an EP additive exhibits good load‐carrying capacity and antiwear properties. The load‐carrying capacity and antifriction performance of PS‐i‐Bu are better than those of the traditional SO; the antiwear performance of PS‐i‐Bu is slightly worse than that of the traditional SO. Scanning electron microscopy and X‐ray photoelectron spectroscopy data show that the PS‐i‐Bu additive undergoes similar tribochemical reactions to the traditional SO during the tribological process. The active element S in PS‐i‐Bu reacts with surface metal mainly to form FeSO4 and/or Fe2(SO4)3 inorganic films.