Abstract: Using MELCOR code, the spent fuel pool (SFP) of 600 MWe nuclear power plant （NPP) was modeled, and the station blackout severe accidents were calculated when the SFP was under normal condition, refuelling condition and the reactor accident condition. The calculation results show that fuel assemblies will melt down and hydrogen will generate, due to zirconium-water reaction, after the half height of fuel assemblies is uncovered. The influence of injection or spray on SFP accidents was analysed, and the results show that SFP accidents will be terminated and the water level of SFP will return up before fuel cladding damage if water is injected or sprayed into the SFP with the boiling evaporation mass rate.