In most investigations on the film cooling performance of turbine endwalls, real annular endwalls were usually simplified into flat endwalls. However, up to now, the influence of flat endwall simplification on cooling performance is seldom considered. In present work, a film cooling experiment was conducted in the hot gas wind tunnel at the University of Science and Technology of China (USTC), in order to validate the conjugate heat transfer algorithm and SST k-omega) turbulence model used in the following numerical simulations. Then a series of numerical simulations was conducted by two endwall models: one is a real annular endwall, and the other is a flat endwall simplified from the first. The cooling performances of the two endwalls were analyzed and compared, and the numerical results reveal the following interesting and important phenomena: (1) Under real conditions, the adiabatic film effectiveness of the annular endwall is generally higher than that of the flat endwall, but at our experimental environment, the overall cooling effectiveness of the annular endwall is lower than that of the flat endwall. (2) Under the real conditions of gas turbine operation, the annular endwall exhibits a "Matthew effect", i.e., better overall cooling performance appear in good film-covered region, but worse overall cooling performance appear in poorly film-covered region. (3) Under the real conditions, the temperature differences between the two endwalls can be larger than 25 K, and the temperature gradient in annular endwall is much higher. With other words, using flat endwall simplification may cause a high risk of endwall lifespan. This is very important for the designers of turbine endwall and users of the experimental data obtained by flat endwalls. (C) 2018 Elsevier Ltd. All rights reserved.