The plant system of a supercritical-water-cooled reactor is the once-through direct-cycle type, where steam-water separators and coolant recirculation systems are not necessary. It is different from those of a boiling water reactor (BWR) and a pressurized water reactor. The supercritical-water-cooled reactor is sensitive to perturbations of the feedwater flow rate because all of the core coolant, driven by the feedwater pumps, flows to the turbines without recirculating core flow. The axial coolant density change is three times larger than that of a BWR. It is necessary to analyze the controllability of the reactor against coolant flow and pressure perturbations to assess the technical feasibility of the reactor. The behaviors of a fast reactor cooled by supercritical water are analyzed for three principal perturbations: change of the control rod position, the feedwater flow rate, and the turbine control valve opening. Based on the step responses to the perturbations, the reactor control system is designed such that the pressure is controlled by the turbine control valves, the main steam temperature is controlled by the feedwater flow rate, and the core power is controlled by the control rods. It is not appropriate to control the pressure by the feedwater flow rate like in a supercritical fossil-fired power plant because of the nuclear thermal-hydraulic coupling. Parameters of the control system are selected by the test calculations to satisfy both fast convergence and stability criteria. Reactor behaviors with the designed control system are stable against the perturbations, although because the plant is the once-through direct-cycle type, the coolant inventory is small. Reactors cooled by supercritical light water are controllable with the described control system.