Abstract: The purpose of the design and layout of the reactor coolant temperature measurement bypass lines and the selection of throttle orifice are to ensure that the transit time of the reactor coolant meets the requirement of the safety criteria（equal or less than 1s）. The Flowmaster simulation model of the temperature measurement bypass lines have been analyzed, and compared to the commissioning data in the nuclear power plant. The analysis results prove that the design and layout of the temperature measurement bypass lines and the selection of throttle orifice are reasonable and correct, and the transit time of the reactor coolant can meet the requirement of the safety criteria.