Abstract: The safety analysis of ATWS accident mitigating system improving design, based on the most serious case--ATWS induced by uncontrolled control rod withdrawal is performed for China Advanced Research Reactor(CARR). Many calculations are done at different maximum withdrawal velocity and with different modes of dropping velocity of control rod(dropping by gravity or dropping within 1.5 seconds). The maximum withdrawal velocity admitted by CARR is found. It is confirmed that the withdrawal velocity of 4 mm/s is selected as the design limit of maximum velocity of control rod in any case. In addition, sensitivity analysis is carried out between realistic analysis and conservative analysis modes. The results show that this setting and the maximum velocity are reasonable and safe for CARR.