In order to solve the false pose of a single circle feature in the monocular vision measurement, a method for eliminating the pose duality of a single circle based on rectangle constrain is proposed. When the camera is calibrated in advance, the images of the circle and rectangle of the target are acquired, and the planes of circle and rectangle are required to be coplanar or parallel to each other. Firstly, the center position of the circle feature and the normal vector of the circle plane can be obtained by the mathematical derivation which has a false solution. Secondly, based on the constrain condition that the real normal vectors between the circle plane and the normal vector of the rectangle plane whose priory condition is unknown are parallel to each other, the false solution can be effectively eliminated. Finally, the real pose parameters of the object can be calculated by the related algorithm, so the real position of the object is acquired. The experimental results show that the absolute error of the object attitude measurement is less than 0.5°, which shows that the proposed method is simple and easy to implement and can eliminate the false pose effectively.