In order to fully utilize the node voltage and line current data of phasor measurement unit（PMU）， a novel algorithm is proposed to predict the perturbed trajectories based on reduced-order approach of reserving voltage-observable nodes. The definition of voltage-observable nodes is presented as well. An accurate method of tracking the network topology is also discussed. Firstly the voltage observability of nodes is analyzed to select the reserved ones according to the perturbed system network topology， then the equivalent dynamic components on the boundary nodes are introduced to reflect the dynamic characteristics of the original system. The type of equivalent component is determined by distribution of dynamic components before order reduction. After all the system components' parameters are acquired， the PMU data are set up as initial values to realize the perturbed trajectories prediction. Theoretical analysis and simulations results show that the method proposed can achieve good effect for system perturbed trajectories prediction.