Abstract: To carry out the accident analysis of Chinese Experimental Fast Reactor, total instantaneous blockage of single subassembly（in short as TIB） is selected as analyzing object. Meanwhile BE+1 experiment of SCARABEE series experiments is selected as validation experiment. During TIB simulation, two-fluid six-equation model is used in sodium boiling period, and the subassembly channel is discretized by intercrossing discretion method radially and axially. In the course of model solution, a optimizing method is put forward aiming at solving some drawback of sub-channel model, which is on the basis of Lame computation factor, and the result is validated on BE+1 experiment, which show a reasonable outcome.