In this study, a simple, label-free, and enzyme-free colorimetric biosensor has been developed for amplified detection of let-7a microRNA (miRNA) on the basis of dual signal amplification strategy. The sensing system mainly consists of four unlabeled hairpin probes termed H1, H2, H3, and H4. Upon sensing of the target miRNA, hairpin H1 is opened. Then hairpin H2 hybridizes with H1 forming H1–H2 duplex and frees the target miRNA that can be recycled to trigger another reaction cycle. In addition, the newly formed H1–H2 duplex hybridizes with hairpin H3, and this triggers the autonomous cross-opening of the two hairpins H3 and H4 through hybridization chain reaction. During this process, numerous split G-quadruplex structures are generated and further associate with cofactor hemin to form massive peroxidase-mimicking DNAzymes. The resulting DNAzymes catalyze the H2O2-mediated oxidation of colorless 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS2−) to the green-colored ABTS•−, inducing a remarkably amplified colorimetric signal. This newly developed sensing system exhibits high sensitivity toward miRNA with a detection limit of 7.4fM and a large dynamic range of 6 orders of magnitude from 10fM to 10nM. Furthermore, it exhibits a good performance to discriminate single-base difference among the miRNA family members and holds a great potential for early diagnosis in gene-related diseases. •A label-free and enzyme-free colorimetric miRNA biosensor has been developed.•Catalyzed hairpin assembly and hybridization chain reaction are perfectly combined.•A low detection limit of 7.4 fM toward target miRNA can be achieved.•The sensing system exhibits a good performance to discriminate single-base difference among the miRNA family members.