The thermal problem in stator windings of a water-cooled 1000-MW supercritical-scale turbo-generator is investigated in this article. The two-dimensional electromagnetic field of a stator slot is calculated, and the eddy current distributions of hollow and solid strands are obtained, as well as the Field coefficients (resistance enhancement coefficient) of strands at different radial positions. Based on the determination of heat source distribution, the three-dimensional coupling simulation of the thermal and fluid fields within a tooth region in the stator are solved, from which the fluid velocities in hollow strands and ventilation duct are obtained, as well as the temperature distribution of the strands. Variations of temperature drop of the main insulation are analyzed at different positions. The temperatures of the insulation layer and water outlet are compared with the experimental results, which show that the proposed model can perform the thermal analysis with reasonable accuracy. Furthermore, temperature fields in stator windings with different blocked hollow strands are analyzed, and the effects of blocked strands locations on local temperature distributions are studied. All these would help not only to predict the degradation and damage of winding main insulation, but also to guarantee the safe operation of the supercritical generator group.