<p>In a canned motor with a spiral tube heat exchanger for a nuclear main pump, if the secondary water suddenly fails, the temperature of the lubricating water and motor winding insulation must not exceed the alarm or permitted value within the 5&#xa0;min response time for safe operation. Therefore, it is crucial to predict the transient characteristics of the temperature distribution of the cooling water and the main components. In this paper, a finite-volume conjugate 3D thermal model is proposed to predict the steady-state/unsteady temperature fields of the small-capacity canned motor for a nuclear main pump. The temperature development characteristics of the main components and the primary/secondary water are analyzed under rated conditions with cold ambient parameters. The predicted temperatures agree well with the measured experimental data a maximum error of + 5.97%. Results show that the temperature difference between the stator/rotor cores and the shielding sleeves becomes smaller than in the steady state. The temperature of the coolant in the nuclear main pump under the motor bottom has an increasing influence on the motor bottom lubricating water temperature over time during transient SWSB failure conditions. It provides references for the thermal design of canned motors for nuclear main pumps.</p>