The airtight blast door with movable threshold is widely used in civil air defense engineering. Due to the low stiffness, the load of the door leaf is quite different between the door with movable threshold and that of ordinary fixed threshold. The current design standardizes have no load distribution computational methods for the airtight blast door with movable threshold, generally designed referring to the ordinary airtight blast door. In order to study the reaction force of the doorframe of the flat concrete airtight blast door, the finite element numerical model of flat concrete airtight blast door with movable threshold is constructed. The dynamic behavior of the flat concrete airtight blast door with movable threshold under nuclear explosion pressure is simulated with ANSYS/LS-DYNA. The variety procedure of the force between leaf and doorframe and the load distribution of the doorframe are presented. The calculation results indicate that the support capability of the movable threshold can be omitted for its weak rigidity. The force between leaf and doorframe fluctuates with the vibration of the leaf. The load distribution of the doorframe is closely related to the deform state of the leaf. The load distribution of the doorframe is mainly decided by the ratio of highness and wideness of the doorframe when the leaf reach quasi-static state.