Abstract: Taking the 3&4 units in Qinshan Second Nuclear Power Plant as an example, the effect of two modes of feedwater isolation on the mass and energy release during a large main steam line break (MSLB) accident, by valve isolation and shutdown of the main feedwater pump, is analyzed by THEMIS code. The thermal-hydraulic phenomenon in the containment is analyzed by PAREO9 code. It is concluded from the results that the isolation mode by valve isolation is more effective to reduce the mass and energy release in MSLB and the increasing of boron concentration in the boron injection tank can also reduce the mass and energy release in MSLB.