As an important biological indicator, the abnormity of the lysozyme level is closely related to many diseases. Herein, we devise a novel ratiometric fluorescence aptasensor for lysozyme based on the controllable excimer formation of a perylene probe, N, N'-bis(6-caproic acid)-3,4:9,10-perylene diimide (PDI) induced by cationic silver nanoparticles (Ag NPs). Binding of lysozyme aptamer with multiple phosphate groups to cationic Ag NPs strongly hinders the formation of excimer, yielding intense monomer fluorescence of PDI probe. With the introduction of lysozyme, the adsorption of aptamer on the surface of Ag NPs will be weakened owing to the specific interactions between aptamer and lysozyme, which greatly facilitates the excimer formation of PDI. Based on the monomer-excimer transition triggered by lysozyme, ratiometric fluorescence aptasensor for lysozyme can be established. A good linear relationship between the ratio of monomer intensity to excimer intensity and the logarithm of lysozyme concentration was obtained in the range of 0.25-15 nM, and as few as 0.25 nM lysozyme could be easily detected. Moreover, excellent selectivity for lysozyme detection and satisfactory results in real sample analysis were also achieved. This work provides an innovative platform for the construction of simple, label-free ratiometric fluorescence sensors towards a wide range of analytes based on perylene probe.