Because of the insulator string creepage distance calculation needs to along one axis section, in this paper, a method of extract axle wire of insulator strings based on the cross ratio invariance is proposed. Firstly, the point cloud data are obtained by Kinect, then on the basis of the Mahalanobis distance discriminant method to eliminate the center data and retainthe umbrella skirt data; Secondly, according to the crossratio invariancebetween the umbrella skirtof insulator strings, the direction vector of axle wire is limited to estimate; Finally, we fitting the centers of the umbrella skirt in crossratio invariance, and the global optimal equation of the central axis of insulator string is obtained. The experimental results show that, our method can extract a the global optimal, accurate and efficient central axis, and it will provide effective support for subsequent creepage distance measurement.