A novel label-free amplified fluorescent sensing scheme based on target-responsive dumbbell probe-mediated rolling circle amplification (D-RCA) has been developed for sensitive and selective detection of mercuric ions. In this strategy, we reported an ingeniously designed dumbbell-shaped DNA probe (D-DNA) that integrates target-binding, amplification and signaling within one multifunctional design. An Hg²⁺–primer DNA (Hg²⁺–p-DNA) was designed to be complementary to the region of D-DNA but with T–T mismatches. The mismatched Hg²⁺–primer cannot initiate the RCA reaction in the absence of Hg²⁺. Stable T–Hg²⁺–T can be formed in the presence of target Hg²⁺, thus it induces the elongation and amplification reaction by a RCA mechanism, resulting in numerous cascade dumbbell probes forming duplex G-rich quadruplex oligomeric structures . Upon addition of N-methyl mesoporphyrin IX (NMM), the signal reporter, a strong interaction between the G-quadruplex and NMM brings about a great fluorescence enhancement. In this way, we successfully converted each Hg²⁺-triggered D-RCA reaction event into detectable fluorescent signals, which were significantly amplified by RCA in an isothermal fashion. This approach can detect 80 fM of mercuric ions, much lower levels than previously reported biosensors, and exhibits high discrimination ability. More significantly, the dynamic range of D-RCA is extremely large, covering 5 orders of magnitude. We also demonstrate Hg²⁺ quantification with this highly sensitive and selective D-RCA strategy in real samples.