To investigate the influence of surface roughness on film cooling performance and the mechanisms， this study proposes a method for generating random roughness. By utilizing low-speed wind tunnel experiments and numerical simulations， the film cooling effectiveness distribution and flow field data of downstream of the laidback fan-shaped hole in the cases of smooth and rough surfaces were obtained. The mainstream velocity was set at 20 m/s， and the blowing ratio was 1.0， 2.0， and 3.0. Results indicate that as the blowing ratio increased， the impact of roughness on the counter-rotating vortex pair in the mainstream region decreased. In all the cases， increased roughness significantly enhanced the velocity fluctuations near the wall and induced numerous vortices， thereby intensifying the mixing process between the coolant and mainstream. The influence of surface roughness on the dimensionless temperature distribution characteristics downstream of the hole was more pronounced at lower blowing ratios， and the influence of roughness gradually increased along the flow direction. At low blowing ratios， increased roughness enhanced coolant dissipation， and reduced film cooing effectiveness by approximately 15%. However， at high blowing ratios， the diffusion effect of coolant caused by roughness played a major role， thereby increasing the film cooing effectiveness by about 20%.