An experimental investigation to measure the effect of mainstream turbulence on blade platform cooling effectiveness within a linear cascade has been completed. Turbulence grids generate the wide ranges of turbulence intensity, from 0.72% to 13%. To simulate the rotating condition of engine blades, velocity triangle analogy has been adopted. Different size of inclined injection hole plates produces swirl motion of purge flow, which makes three different swirl ratios of 0.4, 0.6 and 1.0. Pressure sensitive paint (PSP) technique is used to obtain detailed film cooling distribution on blade platform. The inlet Reynolds number is 250,000, and coolant to mainstream flow rate (MFR) is 1.0%. By using CO2 as a foreign gas, density ratio 1.5 is obtained. Results show the existence of pressure gradient between suction side and pressure side. Horseshoe vortex and passage vortex sweep coolant remarkably, which results in poor film cooling coverage on the blade. Furthermore, the strength of the two vortexes increases for higher rotating conditions. However, more coolant can cover the platform by increasing turbulence intensity because turbulence can reduce the strength of vortex. Especially, film cooling effectiveness increases significantly when turbulence intensity increases from 0.72% to 3.1%.