Abstract: In order to predict the rationality of thermometers setting on hot leg of Generation Ⅲ Advanced Reactor,the hot leg temperature mixing and measurement characteristics was studied for conditions with different core outlet temperature and flowrate distributions, using computational fluid dynamic (CFD) analysis technology, with model consist of volume from core outlet to location of thermometers on hot legs. The results show that obvious temperature stratification of the coolant in hot legs exists, while the deviation of the average temperature measured by the thermometer is relatively small compared with the average temperature of the coolant section. Therefore, the thermometer setting on hot legs for Generation Ⅲ Advanced Reactor is reasonable and the temperature of coolant can be effectively measured.