Abstract: The point kinetic model and the heat transfer model of the reactor system analysis code RELAP5 MOD4.0 were modified, which provided the capacity to simulate the dynamic characteristics of the liquid lead-bismuth cooled sub-critical reactor. The response characteristics of the fuel rods in the sub-critical reactor of China Initiative Accelerator Driven System (CiADS) under beam transient were simulated by the improved code RELAP5 MOD4.0. The variation of stress of the fuel cladding in CiADS under beam transient was analyzed by ANSYS17.0 code. The results show that the shorter the time of beam trip the slower the temperature of the fuel rod recovery. The fuel cladding will not melt in the possible overpower event of the beam. The change of temperature difference between inner wall and outer wall of fuel cladding is the main factor of stress variation. The beam transient will not cause stress damage to the fuel cladding in CiADS sub-critical reactor.