The monocular vision-based navigation algorithm for spacecraft is addressed in this paper. Based on dual number, coupled dynamics equations of relative position and attitude are derived, which can describe the coupling effect between the rotational motion and translational motion. The measurement model is proposed on the basis of the projection on the 2-D plane of the line fixed on the target spacecraft. And the Extended Kalman Filtering method is applied to the estimation problem. Numerical simulations are performed to demonstrate that the proposed approaches can provide accurate and fast estimation of relative position and attitude.