The invention relates to a grid-connected load control method for a nuclear power unit and a system thereof. A first steam command is generated based on a feedback signal of the nuclear power unit grid-connected, a turbine of the nuclear power unit is controlled for start, the nuclear power unit grid-connected is enabled to quickly enter an operation state in an open loop control mode at the initial stage, and the actual power of the turbine of the nuclear power unit is facilitated to approach the threshold in short time. Whether the nuclear power unit enters a closed loop control mode is determined through the comparison result of the actual power of the turbine and the threshold, the actual power of the turbine can be stabilized at the threshold when the nuclear power unit grid-connectedis loaded, a nuclear reactor reactor-turbine power severe mismatching phenomenon caused by violent fluctuation of the actual power can be avoided, and a nuclear reactor loop overcooling problem caused by severe mismatching can be further avoided. The method is advantaged in that strong technical support can be provided for normal operation of the nuclear power unit by the constructed grid-connected load control system.