In this work, we have found for the first time that the fluorescence of rhodamine B (RhB) would be dramatically reduced after it bound to hemin/G-quadruplex and reacted with •OH. Based on this finding, we have designed a colorimetric and fluorescent dual-mode sensing platform for visual detection of hydrogen sulfide (H2S). The constructed sensor is based on the formation of dsDNA and the G-quadruplex structure by the cytosine-Ag+-cytosine mismatch, causing H2O2-mediated catalysis to oxidize ABTS or RhB to induce a colorimetric or fluorescent change. In the presence of H2S, the solution color for colorimetric and fluorescent assays would change from dark green to pink and from green (fluorescence off) to bright yellow (fluorescence on), respectively. This dual-mode assay showed high selectivity toward H2S over other interference materials with a low measurable detection limit value (below than 2.5 μM), and it has been successfully applied to H2S visual detection in real samples. Moreover, the dual-mode sensing strategy presented an excellent reutilization character both in colorimetric and fluorescent assays. This method was employed as a label-free, simple, fast, and equipment-free platform for H2S detection with high selectivity and reusability. This work realized naked-eye detection both in colorimetric and fluorescent analysis at a lower concentration of H2S, demonstrating a promising strategy for on-site visual detection of H2S.