Abstract: To alleviate the thermal stress and fatigue induced by thermal layer separation and improve the erection conditions,the modification,enlarging the angle between surge line piping and primary piping and installing the pressurizer at a higher level,has been taken into account in the new design based on M310 model.This modification can bring the changes of the internal structure arrangement of reactor building and lead to a set of different seismic responses to some extend.A new structure model to account for the modifica-tion is constructed.The seismic system analysis is conducted based respectively on the new and old model,and the results are compared with each other so as to evaluate the differences of seismic responses in two cases.Some conclusions can be provided as reference to evaluate the feasibility of this modification.