The typical third generation pressurized water reactor nuclear power plants trigger safety injection signal to actuate the emergency core cooling system based on the saturation margin of coolant in hot legs of primary circuit, i.e. ΔPsat in hot legs, which represents the difference between saturation pressure and actual pressure in hot legs. In order to identify the risk of spurious safety injection during falling the reactor back to safety shutdown state after steam generator isolation following an accident on secondary side for which safety injection is unnecessary, this paper analyzes thermo hydraulic phenomenon in the hot legs where the steam generators are isolated. By selecting reasonable monitoring parameters, the risk monitoring curve of spurious safety injection, which clearly marks off the risk zone of spurious safety injection, is plotted based on the actuation logic of safety injection. The curve has been integrated into the post-accident operating state monitoring and tracing system for primary circuit, and has been verified and validated thanks to the full scope simulator. It is concluded that the risk of spurious safety injection can be dynamically and effectively monitored following the accidents on secondary side, so that operators can timely take appropriate measures to avoiding spurious safety injection and fall the reactor back to safety shutdown state smoothly.