Monocular optical sensor is a popular configuration for the relative navigation of small satellites. To address the challenge of unobservable states in long-range, angles-only relative orbit determination of non-cooperative targets within near-circular trajectories, we propose a new kinematic model. This model leverages sequential measurements from a monocular optical sensor to enhance estimations of relative orbit elements. Through the previous optical image, the current target position can be predicted, correcting the current estimation results through sequential measurements. The observability of the proposed method is analyzed through the observability matrix. The system satisfies the observability requirements when obtaining the measurement information of three consecutive moments. Simulation results indicate that the monocular vision-based relative orbit determination method has a higher accuracy than that of the traditional angles-only method.