Abstract: For the improved three loops generation Ⅱ NPP, the severe accident caused by large break of spent fuel pool cooling pipe was calculated by using MECLOR1.8.6, and the progression and primary phenomena were also analyzed and researched, such as spent fuel heatup, fuel melt and hydrogen production etc. The results indicate that the severe accident process of spent fuel pool is relatively quite slow, but the spent fuel melt and hydrogen risk of combustion or detonation could still finally cause the release of radioactivity to the environment. Furthermore, the efficiencies of the emergency injection strategy and hydrogen risk management strategy used in spent fuel pool severe accident management guidelines were analyzed, the time windows of these strategies implemented during severe accident were obtained, and consequently the important support can be provided for the development and application of severe accident management guidance for similar plants.