The flows close to a high-pressure turbine endwall are highly three-dimensional and unsteady. These flows increase the aerodynamic loss and augment the heat transfer. In this paper, the unsteady flows and film cooling characteristics of an endwall were studied by using the Detached eddy simulation (DES). First, the unsteady flows and film cooling characteristics of a slot on a flat plate were studied at three different coolant mass flow ratios. A comparison of the experimental and numerical results showed that the DES offered an improved ability to capture the distribution of film cooling effectiveness. The aerothermal characteristics of the interrupted slot on a vane endwall were then studied. By analyzing the unsteady flow as well as the mixing process, it was found that the shedding vortices produced by the step lip played a dominant role in the film cooling distribution and the formation of the horseshoe vortex. The coupling of the shedding vortices with the horseshoe vortex had an important influence on the film effectiveness on the endwall. Finally, the effect of the axial distance between the interrupted slot and the vane leading edge on the formation of the horseshoe vortex was further studied. The instantaneous vorticity contours showed that varying the axial position of the interrupted slot affected the development of the horseshoe vortex. As the axial distance increased, the horseshoe vortex disappeared periodically and then reappeared. As the axial distance decreased, the horseshoe vortex retained a steady pattern upstream of the vane leading edge.