Abstract: The lubricating water temperature of the lower guide bearing is not more than 95℃ within 300 s of the secondary water supply outage (SWSO) fault under the extreme thermal ambient parameters of the rated (thermal rated) conditions, which is an important index for the product of canned motor for the nuclear main pump. It is very important for the thermal safety of the motor to study the calculation method of the temperature field and the evolution characteristics of various parameters with time. First, the finite volume method is used to carry out the fluid-solid coupling simulation of flow and temperature field, and the boundary conditions are based on the inlet and outlet test data measured in the steady state and transient state of the motor SWSO fault under the cold ambient parameters of the rated (cold rated) conditions in the north factory. The error between the simulated and experimental values is +5.97% at 300 s. The model is proven to be accurate. Then, the transient temperature field of the SWSO under the extreme ambient parameters of the thermal-rated conditions is numerically inverted. The results show that there are two stages in which the characteristics of secondary cooling water flow velocity and convective heat transfer coefficient between secondary water and spiral groove wall variation with time are obviously different within 5 min in the SWSO state. The value of h decreases from 7 808.06 W/ (m²⋅K) at t = 0 s to 480.07 W/(m²⋅K) within 13 s, and then h changes little. When the ambient parameters are extremely high under the rated operating conditions, the lubricating water temperature of the lower guide bearing is much lower than the alarm value of 95℃. It provides a reference for the thermal safety calculation of the bearing lubricating water of the nuclear main pump canned motor.