The invention relates to a method for recovering a valuable metal element in waste phosphor by using phosphonium ionic liquid. The method comprises the following steps: collecting phosphor in a waste fluorescent lamp and taking as an initial raw material, using a strong alkaline compound for high temperature roasting to prepare an alkali fusion object, directly using nitric acid for leaching to obtain an insoluble substance leached by water and an acidic material liquid, wherein acidity of the material liquid is 0.1-3.0mol/L, a difunctional ionic liquid extractant [P6,6,6,14][P204] or [P6,6,6,14][P507] as an extractant, isooctyl alcohol is taken as an additive, a diluent selects n-heptane or kerosene, under condition of weak acid, using coexisted mass aluminium nitrate salt as a salting-out agent, extracting and recovering the rare earth element in a phosphor leachate, and then acquiring an organic extract loaded with rare earth ions and a raffinate aqueous solution with rich aluminium nitrate salt. According to the invention, valuable metals in the waste fluorescent lamp can be recovered and used, the process flow is simple, and the method has the advantages of and friendly environment.