By monocular vision, pose measurement of circular feature exists two possible solutions, and the circle radius is required to be known in advance. To overcome these problems, a line structured light vision system composed by a single camera and a line structured light is established to measure the relative position and attitude of an unknown radius circle. Firstly, two possible solutions of the circular feature are calculated according to the constraint of the circle projection in the image plane. Secondly, two intersection points between the line structured light and circular feature are determined in the image plane by image processing algorithm. The ambiguity can be eliminated by the constraint that the intersection points are on both circular plane and line structured light plane, and then position and attitude of the circular feature can be determined. Finally, the mathematical simulation model is built and the measurement error factors are analyzed. The simulation results show that the measurement method is effective and feasible.