Growing evidence suggests that exosomes-encapsulated miRNAs detection is of paramount significance in early diagnostics of cancer due to protection from degradation by ribonuclease. However, exosomal microRNAs with low abundance and subtle variation have restricted their clinical application. Herein, an electrochemical biosensor for highly sensitive detection of exosomal microRNAs has been fabricated based on the double signal amplification strategy. Our proposed amplifier consists of two steps: target miRNA cyclic signal amplification induced by strand displacement reaction (SDR) and subsequent deposition of silver nanoparticles induced by streptavidin-biotin interaction. Consequently, this method shows ultrahigh sensitivity to detect miRNA. Taking miRNA-21 in exosomes as a model analyte, a detection limit of 0.4 fM (S/N = 3) can be obtained. Meanwhile, the method is relatively simple and low-cost without the requirement of enzyme, which has been applied in biological samples successfully. Therefore, our miRNA assay method has shown great promise as molecular tool in the detection of exosomal miRNA and could be widely used in clinic in the future.