The invention discloses a method utilizing a magnetic field to improve the arsenic-removing efficiency of iron-manganese composite oxides. The provided method comprises the following steps: applying a magnetic field on the periphery of an iron-manganese composite oxide fixed bed, and then making water containing As (III) and As (V) go through the fixed bed to purify the water; or adding iron-manganese composite oxides into water containing As (III) and As (V) in a complete mixing reactor, and applying a magnetic field on the complete mixing reactor to trigger the rapid reactions between iron-manganese composite oxides and As (III)/As (V) so as to purify the water. The provided method accelerates the speed of MnO2 reduction reactions. A stable and constant magnetic field can promote the reduction of iron-manganese composite oxides, at the same time, the As (III) in the polluted water is rapidly oxidized into As (V), and then As (V) is adsorbed and removed. The magnetic field is utilized to improve the efficiency of iron-manganese composite oxides on removing arsenic in water, and the provided method can be used to remove arsenic in various waters, such as underground water, surface water, and the like.