In this study, the metering and diffuser length effects of a laidback fan-shaped (LBF) hole on the film cooling effectiveness (FCE) were investigated. The FCE was measured using the pressure sensitive paint (PSP) technique, with a density ratio (DR) of 2.0 and five blowing ratios (BR). The experimental results indicated that the FCE increased as the diffuser length increased. However, when the metering length ratio increased beyond the critical length, the coolant was shifted to a specific side of the diffusion section, resulting in a lower FCE. A computational analysis was performed to investigate the coolant shift at longer metering lengths. Numerical results indicate that the center velocity of the cylinder section is higher for the longer metering length cases, which leads to a large separation bubble in the diffusion section, these resulted in the increase of the flow unsteadiness and instability at the hole exit.