The preformed flat wire winding is widely applied in doubly salient brushless DC generator with a rectangular slot structure due to the high slot filling factor and processing efficiency. The windings are commonly identified as the hottest component. The slot leakage is serious in the doubly salient structure machine, which increases the AC copper loss and thermal load of flat wire winding. Hence, predicting and lowering windings temperature are crucial tasks in the design of a doubly salient brushless DC generator with a flat wire winding. This article proposes the thermal modeling method of flat wire winding, where uneven AC copper loss distribution and thermal resistance between stator and winding are considered. Besides, the thermal model is simplified by canceling the local coordinate system. The characteristics of the temperature distribution of flat conductor winding under water-cooled and fan-cooled conditions are analyzed. Since the thermal resistance between stator and winding prevents the transfer of heat from the winding to the stator, the fan-cooled method is a better choice for the flat conductor winding. Finally, the thermal resistance measurement experiment is the design, and the fan-cooled brushless DC machine with flat conductor winding is developed. The analysis is verified through experiment.