In order to locate obstacles on high-voltage transmission line fast and accura tely and cross them effectively,an algorithm for monocular vision distance measurement in inspection robot is proposed.First,according to position and pose of camera to the transmission line,a rough estimation of dip angle of the transm ission line in images from video frame is acquired.And then the parameters of both edges of the transmission line are detected fast from coarse to fine in which down-sampling and two-stage Hough transformation are used based on the estimated angle.At last,th e distance between the nearest point on the line in image and camera lens can be measured in advance,and according to the ratio of d ifference of vertical axis on the edges line of the point to that of the positio n of the obstacle,the distance between the obstacle and camera lens can be gaine dthen ,after revising,the distance along the transmission line between these t wo positions used in navigation for ro bot can be gained.Tests results show that the algorithm can achieve such accuracy as measurement error less than 5% within 5000mm.At the same time,the running speed of the algorithm is very fast and the processing time is less than 20ms.