Abstract: Tianwan Units 3 and 4 nuclear power units face the risk of nuclear power over-limit during the upward regulation of primary frequency regulation. After the commissioning of Heqi No.1, the units were converted from pure power generation to combined heat and power (CHP) mode, with more complex operating characteristics. This paper analyzes the units" primary frequency regulation configuration and nuclear power constraints under different conditions, studies the control strategy for locking primary frequency regulation actions, ensures nuclear safety through technical measures, and provides a reference for nuclear power control design in similar nuclear power units and industrial steam supply projects.