Abstract: In order to better understand the process and consequences of nuclear power plant（NNP）spent fuel pool severe accident,severe accident analysis model based on ASTEC code for one of third generation NNPs spent fuel pool is established.According the model,loss of cooling（SBO）and loss of coolant inventory with loss of cooling（SBO+LOCA）are analysed in different conditions that are normal operation condition,normal refuelling condition and abnormal refuelling condition respectively.The results show that the progression of accident in spent fuel pool is quite slow and there is a lot of time to respond and mitigate.However,the hydrogen production is extremely high and the radioactive releases are extremely high which is un-acceptable to workers and public after the fuel assemblies are uncovered and damaged. No mitigation measures are in general available to handle such severe consequences.Moreover,the features of ASTEC code to model the spent fuel pool severe accident is also discovered during this study.