Disclosed is a coolant with dispersed neutron poison micro-particles, used in a supercritical water-cooled reactor (SCWR) emergency core cooling system. Since the neutron poison micro-particles are uniformly dispersed in the coolant of the emergency core cooling system for a long period time, their fluidity is not lowered even though the polarity of water is changed in a supercritical state. Therefore, the neutron poison micro-particles absorb neutrons produced from nuclear fission in a nuclear reactor core. Accordingly, the neutron poison micro-particles can be appropriately used as a means for controlling neutrons and stopping a nuclear reactor in the SCWR emergency core cooling system.