Abstract: Demonstration Fast Reactor sets up eight once-through steam generators in each circuit, and based on that, two designs of feed water control system are proposed. The first is to control the total water in each circuit, and the second is to control the water of each module. The simulation models of the two feed water systems are built, and the process of sodium temperature and steam superheat at evaporator outlet is analyzed. The results show that the first design scheme helps to stabilize the steam superheat at evaporator outlet, and the second scheme facilitates the control of the sodium temperature at evaporator outlet.