The invention discloses an icing degree assessment method based on insulator image umbrella stretch-out. The method mainly comprises the following steps that S1 image acquisition is performed: insulator images before and after icing are acquired by using a fixed camera; S2 the images before and after icing are segmented by using a GrabCut graph theoretic approach; S3 a convex hull and a convex defect between every two adjacent insulators in the contour of S2 are solved; S4 an umbrella stretch-out equivalent calculation method is performed; and S5 the insulator icing degree is estimated by the change of insulator umbrella stretch-out before and after icing. The icing and bridging situation of each part of the insulator strings can be detected by the change percentage of umbrella stretch-out before and after icing through the images, and the degree of severe icing can be estimated so that the visual, economical and effective insulator icing degree assessment method is provided.