The invention relates to a reactor core separation method for a molten metal cooling reactor. The method is implemented mainly by an inner reactor core, an outer reactor core, an upper gird plate inside the reactor core, an upper grid plate outside the reactor core, a lower grid plate inside the reactor core, a lower grid plate outside the reactor core, an inner reactor core coaming, an inner reactor core support column and inner reactor core guide columns, wherein a fuel component is fixed through clamping of the upper and lower grid plates. According to the reactor core separation method provided by the invention, the buoyancy of molten metal to the fuel component is utilized, a reactor core can be lifted through the buoyancy after constraint of the fuel component is released, and thus the purposes that the reactor core is separated and a reactor is closed are achieved. By adopting the method provided by the invention, a redundancy auxiliary system serving as a reactor control rod for stopping a reactor can be also independently used, and then the purpose of closing the reactor can be achieved.