The invention provides a sag measurement method for an overhead transmission line based on binocular vision, and the method includes the steps: (1), constructing a binocular vision system, and establishing a three-dimensional space coordinate system; (2), calibrating the binocular vision system to obtain internal and external parameters of the binocular vision system; (3), collecting and correcting onsite transmission line images; (4), carrying out the line selection and centerline extraction; (5), performing the feature points matching of lines in left and right views; (6), performing the projection of the feature points of the lines on a sag measurement plane; (7), calculating the sag. According to the invention, two cameras of the binocular vision system just need to capture two imageson a site, and then the sag value can be obtained quickly, so the onsite workload is small. Moreover, the method exerts no requirements for a photographing place, and is high in applicability.