Multirow film cooling is widely used in gas turbine airfoils cooling to cope with extremely high inlet temperature. The film cooling effectiveness of diffusion slot holes was measured in multirow arrays at a flat plate experimental facility using the pressure sensitive paints (PSP) technique. Emphasis was focused on the influence mechanism of mainstream turbulence intensity (Tu) and coolant-to-mainstream density ratio (DR) on film cooling effectiveness of multirow arrays with wide exit holes. The diffusion slot hole with exit width of 3.7D and 4.0D was, respectively, tested in three staggered arrays of P/D = 6 and S/D = 30, P/D = 6 and S/D = 20, and P/D = 9 and S/D = 20. Four blowing ratios varied from M = 0.5 to 2.0. The baseline condition (Tu = 3.5%, DR = 1.38) was compared with an increased Tu condition (Tu = 8.4%) and a decreased DR condition (DR = 0.97). The results showed that in two P/D = 6 arrays, increasing Tu decreased the multirow effectiveness considerably under all blowing ratios, but increasing Tu prominently restrained the film coalescence. The influences of Tu were more intense for the arrays with larger exit width holes and larger S/D. Decreasing DR increased multirow effectiveness obviously. The influences of DR tended to consistent for all arrays and the influences were relatively weak for high blowing ratios. In the P/D = 9 array, the superiority of large exit width hole disappeared completely, regardless of Tu and DR variations.