Observable conditions are given for tracking and observing non-cooperative targets under space-based single-star angular-only tracking. To solve the degradation of the estimation accuracy of unscented Kalman filtering (UKF) when the observable degree is low, this paper adopts an improved unscented Kalman filtering (IUKF) algorithm based on observable degree. With the introduction of the energy scaling parameter, the filter gain covariance matrix can be adjusted online according to the size of the energy scaling to modify the weights of state prediction and system observation in real time. Mathematical simulations show that the application of this method to low-orbit observation satellites can improve the estimation accuracy and shorten the convergence time, which can be better applied to near-Earth satellite observation missions with short rendezvous times.