The invention relates to a system and a method for heat standby of a nuclear power started electric boiler. The system comprises a started electric boiler, a superheater of the started electric boiler, an auxiliary steam header, an auxiliary steam user, a shaft seal system, a deaerator of the started electric boiler, a normal steam source of the auxiliary steam header, a steam trap and a water supplementing system of the started electric boiler and the like. The method comprises the following steps: in a starting stage of a nuclear unit, supplying steam to the deaerator, the auxiliary steam system of the unit and the shaft seal system by steam generated by the started electric boiler; reducing output of the started electric boiler after the normal steam source of the auxiliary steam headersupplies steam to the auxiliary steam header; and starting the superheater of the electric boiler and a related pipeline system thereof to enter a heat standby mode by the shaft seal system when thestarted electric boiler does not supply steam, supplying steam to the auxiliary steam header by the normal steam source of the auxiliary steam header and supplying steam to the deaerator of the started electric boiler, the superheater of the started electric boiler and the started electric boiler by the auxiliary steam header, wherein the started electric boiler, the deaerator of the started electric boiler, the superheater of the started electric boiler and the related pipeline system thereof are in heat standby states.