The invention discloses a method for detecting the ageing of a composite insulator umbrella skirt. The method comprises the following steps: selecting a composite insulator high-pressure side umbrella skirt as a sample; performing slow positron beam measurement on the sample, so as to obtain a Doppler broadening spectrum distributed along with positron incident energy; collecting an S parameter of the Doppler broadening spectrum; judging whether the S parameter is in a preset parameter range or not; if so, judging that the composite insulator umbrella skirt is aged. The detection method disclosed by the invention is small in sampling amount, and simple and convenient in test method, and information of substances can be very sensitive to defect; the detection method can serve as a scanning probe of a silicon rubber surface structure and can be applied to preliminary identification and evaluation of the ageing of the composite insulator umbrella skirt.