Observability of optical monitoring data based on single space-based satellite is poor when the orbit is improved, iteration cannot be converged and even normal equation would be ill-conditioned, which would cause failure of cataloging and orbit determination of a large number of space targets. Features and difficulties of application of single-satellite optical monitoring to determine the orbit of space target were analyzed. Then the rank deficiency of orbit determination based on space-based monitoring and the observability of orbit determination of single-satellite optical monitoring were studied. On the basis of priori orbit information, a virtual ranging model was established. An orbit improvement method that combined virtual ranging and space-based angle measurement data was put forward, which improves the observability of optical monitoring data orbit determination system based on single space-based satellite. The verification was implemented on the basis of measured data of more than 400 targets monitored in 40 days in 2015 by the first space-based monitoring test satellite of China. After virtual ranging was added, success rate of orbit determination is increased from less than 10% to 90% above and the precision of orbit determination is improved as well.