The projection position of tube axis should be calculated accurately when the tube axis is measured using stereo vision. Previous methods for measuring tube axis approximate the centerlines of image contours as the axis projection. This assumption exists errors. In order to obtain the exact projection position of tube axis and to improve the measurement precision of tube axis, the proposed method studied the imaging characteristics of tubes. Firstly, a perspective projection model for any cross section of tube was established to explore the geometric relationship between the contour points of the cross section and their corresponding projection points on space axis. Furthermore, a way to locate the precise projected position on image planes of points laying on axis was presented. A binocular vision system was constructed to measure an aerospace tube. Compared with classical approaches, the repeatability accuracy of the proposed method is same as the former, while the measurement accuracy of the presented method increased by 14.5% on average due to precisely calculating the projection position of axis. It is shown that the proposed method is correct in calculating the projection position of tube axis and can effectively improve the measurement precision of tube axis.