Return voltage method is a nondestructive testing technique for studying the insulation aging state of power transformer. The return voltage contains abundant information about dielectric relaxation response reflecting the insulation aging status of transformer. In order to give full play to the advantages of return voltage method in diagnosing transformer oil paper insulation aging, a new method is proposed, which evaluates the insulation aging state of transformer using the quantity of relaxation charge. The quantitative relationship between the relative quantity of relaxation charge and the furfural content in the transformer oil is given. First of all, the return voltage curve is processed with the timedifferential unfolding and the relaxation response function of the return voltage is solved. Secondly, the relative quantity of relaxation charge in the measurement process of return voltage is calculated using the relationship between the quantity of relaxation charge and the relaxation response function. Finally, the relationship between the relative quantity of relaxation charge and insulation aging state is analyzed. Analysis results show that the quantity of relaxation charge is very sensitive to the aging of transformer oil paper insulation. There is a good linear relationship between furfural content and the relative quantities of relaxation charge Q1000 and Q500. The relative quantity of relaxation charge can be used for the quantitative evaluation of the aging of transformer oil paper insulation.