The invention provides a method for evaluating the insulation aging degree of an oil-immersed type power transformer. By comprehensive consideration for multiple aging parameters such as the operating temperature, the content of gas in oil, the content of furfural and the concentration of corrosive sulfide of the transformer, the accuracy of a discrimination result is guaranteed. The method mainly comprises the following steps: detecting the content of gas dissolved in the oil and calculating the polymerization degree; measuring the content of the furfural in the oil and calculating the polymerization degree; determining the operating temperature and calculating the polymerization degree; measuring the initial content of dodecanethiol and DBDS in the oil and calculating the influence and the action on the decreasing rate of the polymerization degree of insulation paper; weighting and calculating the polymerization degree calculated according to all aging characteristic quantities, thus obtaining the polymerization degree of the insulation paper, and evaluating the insulation-aging condition. The method provided by the invention has the advantages that the basic principle is simple, the operability is strong, and the problems of inaccuracy and limitation of aging evaluation by single characteristic parameter are effectively solved; multiple parameters are introduced for combined evaluation of the aging, so that the accuracy and the truth of the result are guaranteed, and the method has important practical application significance for evaluating the insulation aging and the service life of oiled paper.