A new approach has been proposed to measure the concentration of phosphate anion in liquid via optical fiber evanescent wave method.A standard single-mode optical fiber is used and its extramural cladding is corrupted by hydrofluoric acid.The optical fiber sensor was put into phosphomolybdate blue solution with different concentrations.The evanescent wave of the fiber would be absorbed partly by the solution.The phosphate ion concentration can be obtained by measuring the optical output intensity of the fiber.The experimental results show that there is a linear relationship between the optical absorbance of the solution and the concentration of phosphor in the concentration range of 0.04-0.4 mg/L with the detection sensitivity of 0.53 L/mg and the linear dependent coefficient of 0.918.The optical fiber sensors we have investigated are compact and deserve the opportunities for realizing the miniaturization of test equipment.