With regard to the non-cooperative actively approaching spacecraft with maneuverability， a threat index calculation model to analyze the threat of the chaser to the target is proposed. We extract the impulse and the time required for rendezvous based on two impulses Lambert algorithm as threat elements after analyzing the mission mode of the chaser. The most threatening route is selected to weigh the impulse and time in order to calculate the comprehensive threat index. Simulation results show that the threat index of the chaser is very sensitive to the changes of orbital elements. Comparing the commonly used relative distance index， the threat index can identify the maneuver intention more quickly according to its trend. Under the complex space situation， the threat index analysis model is able to predict the threat of actively approaching spacecraft so as to take timely protective measures and improve the survivability of satellites.