Abstract: The shutdown physics process of CFBR-Ⅱ was analyzed based on dynamic function in point reactor model and principle of neutron multiplication. It is found that the plateau power is not determined by preloaded reactivity because the intensity of delayed neutron counteracts neutron multiplication. The plateau power and the power within 350 s after burst were measured by two ¹⁰B ionization chamber systems. The respective powers in shutdown process are 3.5 MW on plateau for delayed neutron, 209 kW when major safety block drops to 5 mm, 4.8 kW when the all rods and major safety block get out and 60 W at 30 s after burst.