Sub-criticality monitoring under post-accident conditions is of great significance for emergency operations in nuclear power plants, as many post-accident physical phenomena may disturb the measurement process of neutron flux outside the reactor, how to quantitatively evaluate the disturbances is therefore becoming a key issue for core reactivity monitoring in post-accident conditions. In this paper, the Monte Carlo model is established based on the actual neutron diffusion process to quantitatively evaluate the disturbance to neutron flux measurement outside the reactor considering the following post-accident physical phenomena: group (N-1) control rod assembly dropped after shutdown, reactor cavity flooding, core cavitation, core draining. The amplification and attenuation factors due to these physical phenomena are obtained, based on which proposals are suggested to realize an accurate diagnosis of the core sub-criticality state and to prioritize the nuclear power control in post-accident conditions, this is essential for operating strategy orientation and fuel clad barrier integrity control in the emergency operation of nuclear power plants.