Silver nano-cubes of about 97 nm side-length were synthesized by reducing AgNO3 with ethylene glycol (EG) and used to modulate fluorescence intensity of Rhodamine B (RhB). The RhB (probe molecules) powder was dissolved into PMMA solution with different PMMA concentrations, which resulted in RhB doped PMMA films with different thickness. The distance between Ag nano-cubes and the fluorescence molecules and distribution of Ag nano-cubes has important influence on the fluorescence emission. The fluorescence spectrums show the enhancement and quenching effect with different thickness of PMMA film. The largest fluorescence enhancement factor (EF) is 56 for the structure of Ag nano-cubes absorbed on the RhB doped PMMA film as the thickness of PMMA is 10 nm. The confocal-images also verified the fluorescence enhancement. The EF is influenced by the distribution of silver nano-cubes on the PMMA film from the confocal-images. So the biggest EF can be obtained by optimizing the distance between silver nano-cubes and the fluorescence molecules or by tuning the distribution of silver nano-cubes, which may be used to single molecule detection. The experiment results have potential application in fluorescence based bio-sensing or bio-imaging.