The invention relates to a method and device for purifying indoor harmful volatile organic compounds. The method for purifying the indoor harmful volatile organic compounds comprises the steps: an air purifier loaded with a photocatalyst is provided; the air purifier is internally filled with a mixed oxidizing agent composed of ozone and hydrogen peroxide; and air containing the harmful volatile organic compounds is allowed to flow through the air purifier, so as to degrade the harmful volatile organic compounds under the action of the photocatalyst. The method not only can effectively inhibit gradual disappearing of the photocatalyst activity during the photocatalytic reaction process, but also avoids secondary pollution brought by use of high-concentration O3 and after-treatment required for O3.