This work presented a simple, sensitive and label-free electrochemical method for the detection of microRNAs (miRNAs). It is based on the boronate ester covalent interaction between 4-mercaptophenylboronic acid (MPBA) and cis-diol at the 3′-terminal of miRNAs and the MPBA-induced in situ formation of citrate-capped silver nanoparticles (AgNPs) aggregates as labels on the electrode surface. In this design, MPBA acted as the cross-linker of AgNPs assembly. Specifically, the thiolated hairpin-like DNA probe was assembled onto the gold nanoparticles (nano-Au) modified electrode surface through the Ag-S interaction. After hybridization with the target miRNAs, MPBA was anchored onto the 3′-terminal of miRNAs through the formation of a boronate ester bond and then captured AgNP via the Ag-S interaction. Meanwhile, free MPBA molecules in solution induced the in situ assembly of AgNPs on electrode surface through the covalent interactions between α-hydroxycarboxylate of citrate and boronate of MPBA and the formation of Ag-S bonds. The electrochemical signal was therefore amplified due to the formation of AgNPs network architecture. To demonstrate the feasibility and analytical performances of the method, miRNA-21 was determined as a model analyte. The detection limit was found to be 20 aM. The viability of our method for biological sample assays was demonstrated by measuring the miRNA-21 contents in three human serum samples. In contrast to other signal-amplified electrochemical strategies for miRNAs detection, our method requires simple detection principle and easy operation procedure and obviates the specific modification of nanoparticles and capture/detection probes.