In the bridge crane payload space positioning system based on monocular vision, in order to solve the problem of matching performance degradation caused by the rotation and tilt of the target template, a real-time bridge crane payloadspace positioning method based on circle detection with vertical gradient direction lines and linear pre-interpolation was proposed. Aspherical target was attached to the top of the payload, and the spherical target was not sensitive to the rotation and tilt when it was detected. First, the spherical target in the Region of Interest (ROI) was detected accurately and fastly by the circle detection method based on vertical gradient direction lines. Secondly, the space coordinates of the spherical target center was confirmed by the space geometry method. And then, the space coordinates were fed back by the method of the linear pre-interpolation. In the contrast experiment with traditional method, space coordinates were transformed to the cable lengths and the payload-swing angles. The experimental results show that the payload-swing angles transformed by this method are more accurate than that of traditional method, and the method can meet the real-time requirement. Moreover, the maximum length measurement error between this method and traditional method is 2.49%, which meets the accuracy requirement.