An Improved A∗ algorithm based on time window is proposed to solve conflict-free path planning problem for the Automated Guided Vehicles (AGV) in flexible manufacturing systems. Based on the planned AGV paths, conflict-free routing was received by utilizing the improved A∗ algorithm in the next AGV routing. The algorithm of improving evaluation function to introduce the turning factors not only avoids conflicts between vehicles, but also makes the path smoother. The dynamic adjustment range of the strategy in the light of AGV performance is suggested to reduce the accumulative error caused by time windows, which can increase robustness and flexibility of the system. The proposed strategy was verified to be effective by experiments.