The invention discloses a method for preparing M-phase VO2 by using an inactivated vanadium battery positive electrolyte. The method belongs to the technical field of vanadium dioxide, and comprises the following steps: (1) adding a reducing agent into the anode electrolyte of the failed vanadium battery to carry out reduction reaction, adding a precipitant into the electrolyte after the reductionreaction is finished to adjust the pH value to 3-6, carrying out solid-liquid separation, and washing the solid product to obtain a black precipitate; and (2) calcining the black precipitate to obtain VO2 powder. According to the method, the inactivated vanadium battery electrolyte is used as a raw material, so that resource recycling is realized, and the preparation cost of VO2 is reduced; meanwhile, the method is simple in process and suitable for industrial application; and the prepared VO2 is high in purity, high in additional value and high in crystallinity, and can be widely applied tothe fields of intelligent windows, photoelectric switches, thermistors and the like.