Resistance spot welding is a very complicated process involving electro-magnetic, thermal, fluid flow, mechanical and metallurgical variables. Since weld nugget area is close and unobservable with experimental means, numerical methods are mainly used to reveal the nugget formation mechanism. So far, heat transfer behaviors in the weld nugget have been systematically studied with traditional electro-thermal model, however, the model cannot model the fluid flow caused by induced magnetic field in the molten nugget. In this paper, a multi-physics model, which comprehensively considers the coupling of electric, magnetic, thermal and flow fields during RSW, is used to investigate the fluid flow and heat transfer laws in the weld nugget. Results show that molten metal in the nugget makes high speed rotation motion in four cores, moreover, the rotation motion significantly changes the profile of the nugget and the thermal field evolution, compared with the traditional electro-thermal model. Because the fluid flow and heat transfer jointly determines the crystallization process and final microstructure, as a result, when predicting crystal growth process, the effects of the fluid flow during resistance spot welding should be considered in order to obtain a more accurate prediction results.