Abstract: The canned primary pump motor is an important part in nuclear power stations,its safe operation and stability is very important to a loop system of the nuclear island.The motor works in high temperature and high pressure conditions,its heat and cooling are complex.According to hydromechanics and heat transfer theory,a physical model of three dimensional fluid and temperature coupled field was established for 5 500 kW large canned motor of nuclear primary pump as an example in this paper.The temperature distribution of the motor under the nominal operating condition was calculated using the finite volume method.Calculation results revealed the regularity of temperature distribution for the canned primary pump motor,and provided the theory basis for cooling structure design and accurate temperature calculation of canned primary pump motors of larger capacity.