A modular electric power device for a data center comprises 1+N nuclear power generation system modules, an auxiliary system module group and a control system module; the nuclear power generation system modules are connected with the auxiliary system module group and the control system module; output ends of the nuclear power generation system modules are connected to the data center; the nuclearpower generation system modules are disposed below the ground; 1+N nuclear power generation system modules are included, wherein the power generation of the N nuclear power generation system modules can meet the power consumption of the data center, and realize uninterrupted power supply to the data center throughout the year; the nuclear power generation system modules have small power and a small size, adopt a passive design, can be completely placed underground and submerged in underground module storage water pools, and realizes passive residual heat discharge through waste heat export devices. The refueling method of the whole reactor core and an integrated head package assembly are adopted to significantly shorten the refueling time, and the resisting ability to external events suchas aircraft impact, terrorist attacks, hurricanes and earthquakes can be significantly improved.