Abstract: The Daya Bay spent fuel pool was chosen as the analysis object and the best estimate system thermal hydraulic code RELAP5/MOD3.3 was employed to investigate the processes in the spent fuel pool in case of LOCA. The results of calculations showed that, the increasing of water temperature in the pool from 52.6℃ up to 100℃ took 6.1 hours, and the evaporation of water volume above the SFAs took 16.6 hours. During this period, if some remedial measures were taken, such as taking the appropriate measures to cool the spent fuel or timely suppling water to the pool, it would be possible to avoid the uncovering of the SFAs.