It is demonstrated that miRNAs exhibit significant regulatory roles in a series of biological processes and associated with diverse human diseases. Herein, we report a convenient fluorescent biosensor for the quantitative determination of miR-21, a key miRNA related to cardio-cerebrovascular diseases. Our proposal involves not only the rational design of single stranded DNA as the probe, successively including a C-rich sequence as the synthetic template of DNA/Ag nanoclusters (DNA/AgNCs), a complementary (Com) sequence to hybridize with the miR-21, and a G-rich sequence to form a complex of G-quadruplex/hemin but also the distance-dependent property of photoinduced electron transfer (PET) between the preformed DNA/AgNCs (electron donor) and G-quadruplex/hemin complex (electron acceptor). In the presence of the target miR-21, the initial flexible single strand Com in the probe turns to the rigid Com/RNA heteroduplexes, and then the PET could be interrupted owing to the extended distance between the electron donor and acceptor, accompanying with the fluorescence quenching and recovery of DNA/AgNCs. Therefore, a fluorescence light-up biosensor for miR-21 could be developed through the monitoring of the degree of fluorescence recovery of DNA/AgNCs. Preferential to other previous PET-based detection methods, we construct the biosensor by utilizing the distance dependent property for the first time and only need to adjust the sequences of Com in different miRNAs assays.