The invention provides a thickening agent used for water plugging, and a water-plugging thin oil densifier prepared from the thickening agent. The thickening agent used for water plugging is composed of a main agent and an auxiliary agent; the main agent is composed of a zwitterionic surfactant and an anionic surfactant; the zwitterionic surfactant is a phosphate ester betaine-type zwitterionic surfactant, the anionic surfactant is a disodium laureth(3) sulfosuccinate anionic surfactant, and the auxiliary agent is a fatty alcohol polyoxyethylene phosphonate nonionic surfactant. The water-plugging thin oil densifier is prepared via mixing the thickening agent with thin oil; the water-plugging thin oil densifier is nontoxic and friendly to the environment, is excellent in biodegradability and compatibility, simple in operation process, and high in selective water plugging ability, possesses high temperature resistance and hypersalinity resistance, and is suitable for thin oil reservoirs; and no base layer damage is caused. The invention also provides a method used for oil well water plugging with the water-plugging thin oil densifier.