Based on a petal-like microstructure model of alloy particles we proposed, the field-regulating characteristics of alloy-based metamaterial films in the wavelength range of 300-800 nm are analyzed. It is found that Au/Ag alloy particles can support a broader resonance band with higher averaged resonance intensities than that of pure silver or gold particles, which, named alloy plasmonic effect, proves to be a universal feature of alloy-based plasmonics. Upon optimizing the coupling interaction between the alloy plasmonic effect and absorption saturation effect within alloy-based multilaminar structures, a broadband electromagnetic wave absorber consisting of a Cu/Al alloy-based composites layer and an aluminum base layer is demonstrated. Furthermore, a generalized method is proposed to evaluate the absorption performance of this kind of plasmonic absorber. The achieved alloy-based absorber proves to be nearly non-iridescent and the quality factor AP throughout the range of 300-800 nm remains higher than 0.8 even if the incident angle increases up to 60 degrees.