A highly sensitive fluorescence biosensing system was designed for the detection of trace amounts of arsenic(III) (As3+) based on target-triggered successive signal amplification strategy. The specific recognition between the target As' and the aptamer sequence results in the release of the blocking DNA to trigger the subsequent signal amplification steps. Exonuclease III (Exo III) -mediated DNA recycling digest process is introduced into the sensing system to generate numerous Mg2+-dependent DNAzymes. After magnetic separation, the active DNAzyme with multiple turnovers could catalyze the continuous cleavage of the fluorophore-quencher-functionalized substrate strands, thus yielding a significantly amplified fluorescence signal for target detection. Due to the synergetic signal amplification of Exo III and DNAzyme, the fluorescent biosensor exhibits ultrasensitivity for As3+ monitoring, with a detection limit of 2 pM. Our established biosensor also displays excellent selectivity toward the target As3+ and has been successfully applied to the determination of As3+ in water samples with satisfactory accuracy. This sensing platform can be developed as a universal approach for the fast, sensitive, and accurate detection of aptamer-binding molecules.