A RETRAN02/MOD5 model was developed for Lungmen ABWR and applied for ATWS transient analysis. Three ATWS events including Main Steam Isolation Valve Closure (MSIVC), Loss of Offsite Power (LOOP), and Inadvertent Opening of all Turbine Bypass Valves (IOTBV) are analyzed in this study. During the first two transients, the vessel pressure is increased as a result of steam flow reduction due to the closure of main steam isolation valves (MSIV5) and Turbine Control Valves (TCV5) respectively. In the third transient, the vessel pressure is reduced because of the open of Turbine Bypass Valves (TBV5) and turns to be increased because of the closure of MSIVs. All of the above transients suffer high neutron flux as a result of void reduction. There are several equipments and procedures to mitigate ATWS transient such as feedwater trip, Reactor lnternal Pumps (RIPs) runback and trip, and the depressurization of relief valves. After the ATWS high pressure signal is initiated and permissive for 180 seconds, Standby Liquid Control system is initiated to inject boron liquid into upper plenum to shutdown the reactor. The results conclude that equipments and procedures mitigate the event effectively and the core is brought to shutdown state.