In order to achieve the goal that unmanned aerial vehicle(UAV) automatically positioning during power inspection ,a visual positioning method which utilizes encoded sign as cooperative target is proposed .Firstly ,we discuss how to design the encoded sign and propose a robust decoding algorithm based on contour .Secondly ,the Adaboost algorithm is used to train a classifier which can detect the encoded sign from image .Lastly ,the position of UAV can be calculated by using the projective relation between the object points and their corresponding image points .Experiment includes two parts .First ,simulated video data is used to verify the feasibility of the proposed method ,and the results show that the average absolute error in each direction is below 0 .02 m .Second ,a video , acquired from an actual UAV flight ,is used to calculate the position of UAV .The results show that the calculated trajectory is consistent with the actual flight path .The method runs at a speed of 0 .153 s per frame .