Control melt flow in tundish is very important for clean steel production. To explore the fluid flow mechanism, the RTD curves and its data, velocity vector fields and streamlines of the molten steel, and the relationship between the RTD curves and flow pattern in a single-strand slab caster tundish with a capacity of 30 tons have been investigated by both hydrodynamic and mathematical simulations. The RTD data and flow field of the original tundish have been studied in a 1:3 reduced scale hydrodynamic model. Meanwhile, the streamlines, velocity vector fields and the RTD data of the ratio of width to length (W/L) in tundish are mathematically simulated. In order to descript the flow pattern better, a new method is proposed to calculate the data of RTD curves with double peaks. The results showed that the RTD curves changed from double peaks to single peak with the increasing of the W/L in tundish. Both hydrodynamic and mathematical simulations results suggest that the W/L in tundish is the most important factor to change the flow pattern actually, that is, the short-circuiting flow disappeared with the increase of the W/H in tundish gradually. Furthermore, we have elaborated the mechanism the RTD curves change from double peaks to single peak. With increasing W/L, the wide-side walls play an important role to retard the short-circuiting flow on the inlet-outlet plane straight towards the outlet. Meanwhile, the dead region and its volume fraction are also the objects of our attention and exploration.