The invention discloses a high-definition-image-based calculation method and system for icing thickness of a transmission line. The method comprises: step one, obtaining a real-time image including a lead and a ground wire of a transmission line; step two, measuring a thickness ratio of a lead to a ground wire in a real-time image at a same scene and determining an icing degree of the ground wire preliminarily; and step three, according to diameter specifications of the lead and ground wire of the transmission line and a practically measured thickness ratio value, obtaining an icing thickness value by using an improved standard ice thickness calculation module. According to the method and system disclosed by the invention, the manual ice observation result is perfected; an error between the manual ice observation result and the practical icing situation can be reduced; the manual ice observation accuracy can be improved; and an accurate information support is provided for de-icing decision making.