As icing transmission lines are harmful to the security of the power system and effective monitoring of icing on transmission lines can prevent serious accidents， an icing transmission lines on-line monitoring method based on 3D reconstruction is proposed. With this method computer binocular vision technology is adopted， the procedure of which is as follows: process two images of an icing transmission line obtained by two cameras in different locations， calculate the depth of the icing transmission line in 3D space by the parallax of the two images， reconstruct the 3D model of the icing transmission lines， and then calculate the thickness and the distribution of the ice covering the transmission lines based on the platform of the 3D model. Reconstruct the 3D models of two groups of measured images by the method proposed and the comparative analysis results show that the method is not only highly accurate and reliable in calculating the thickness， but also can reflect the ice thickness distribution.