In order to overcome the mismatching between traditional additives and DLC coatings in solid-liquid lubricating system. Cu nanoparticles modified with diisooctyl dithiophosphoric acid (NPCuDDP) were prepared and used as nano-additives in DLC/PAO solid-liquid lubricating systems. Four kinds of DLC coatings including amorphous carbon (a-C) and amorphous carbon doped with Si (a-C(Si)), Al (a-C(Al)) and H (a-C(H)) respectively, with a hardness range from 22 GPa to 7 GPa were used to study the tribological mechanism and adaptability of NPCuDDP to the hardness and composition of DLC coatings. It was found that NPCuDDP reduced the COF of DLC coatings/PAO systems by 19%-22% regardless of their hardness and composition and made the wear rate of all DLC coatings decrease by 2-3 orders of magnitude. While the composite of tribofilm was decided by the doped elements in DLC coatings. The doped element H inhibited the oxidation of copper in tribofilm, and led to the lowest wear rate of a-C(H) coating. The doped element Al in a-C(Al) facilitated the enrichment of phosphates on friction surface. The hardness rather than composition of DLC coatings determines whether traditional additive ZDDP can form tribofilm. ZDDP can only form Zinc polyphosphate tribofilm on the steel balls rubbed with a-C, a-C(Si) and a-C(Al) coatings whose hardness was over 12 GPa. The friction coefficient of the system increases greatly because of the high strength of ZDDP tribofilm. The high activity of soft metal nano-particle in NPCuDDP urged the formation of tribofilm on friction surface easily and produced a significant antifriction and antiwear effect, and is expected to replace traditional small molecule additives as the high-efficiency special lubricating oil additives for DLC coatings solid-liquid lubrication system.