The invention relates to a method for preparing vanadium pentoxide and belongs to the field of fine chemical engineering. The purpose of preparation of the vanadium pentoxide is achieved. The method includes the following steps of a, re-dissolution, b, crystallization, c, water addition for dissolution, d, vanadium settlement, and e, calcination, wherein in the re-dissolution step, ammonium polyorthovanadate is dissolved in a sodium hydroxide solution, so that a re-dissolution solution is acquired; in the crystallization step, the re-dissolution solution stands still for 20 h to 40 h at the indoor temperature and filtered, filtrate is acquired and stands at the temperature from 2 DEG C to 5 DEC G so as to be crystallized, and sodium metavanadate solid is acquired; in the step of water addition for dissolution, water is added to the sodium metavanadate solid for dissolution, so that a dissolved solution is acquired; in the vanadium settlement step, ammonium sulfate or ammonium sulfate is added to the dissolved solution, a reaction is conducted at the temperature from 45 DEG C to 65 DEG C for 0.5 h to 2 h, the solution is filtered, and ammonium metavanadate solid is acquired; in the calcination step, the ammonium metavanadate solid is calcined, so that vanadium pentoxide is acquired. According to the method, the vanadium pentoxide with high purity can be obtained, the process is simple, and operation is convenient; in the preparation process, vanadium pentoxide can be prepared by adding a small amount of sodium hydroxide and ammonium salt, raw materials are saved, and the method accords with the industrial policies of energy conservation and emission reduction.