We demonstrate that the amplified spontaneous emission (ASE) in an Ag-backed red-fluorescent-dye-doped polymer film can be controlled by the effect of the film thickness. Optical losses associated with the metallic contacts necessary for charge injection, an obstacle to the development of an electrically pumped organic solid-state laser, may be possible to be reduced by increasing the gain medium layer thickness. The study of ASE characteristics of Ag-backed 4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)-doped polystyrene (PS) films with different thicknesses shows that increasing the film thickness can reduce the influence of the Ag layer. The threshold, gain, and loss of the device with a thickness of 800 nm are comparable to those of a metal-free device. Our findings demonstrate that the Ag-backed DCJTB:PS film can still be a good organic gain medium material for the fabrication of solid-state lasers, when the thickness of the DCJTB:PS layer increases to an appropriate value.