The viewpiont of molecular design for water soluble lubricant additives is tested and verified. In accordance with the viewpoint, carboxyl acid derivants of O,Odialkyl-phosphorodithioate [(RO)2P(S)SR' COOH] were designed and synthesized Tribological tests were carried out. The results show that it is a new effective water soluble lubricant additive with the multifunction of both oilness agent and extreme pressure agent.