Herein,we propose a novel photoelectrochemical (PEC) biosensor for dual microRNAs (miRNAs) highly sensitive and simultaneous biosensing based on strand displaced amplification (SDA) reaction.The recognition of HmiR-21 and Hlet-7a by microRNA-21 and let-7a leads to their change in hairpin structures,subsequently initiating the immobilization of abundant CdS quantum dots (CdS QDs) and methylene blue (MB) based on SDA reaction.The immobilized CdS QDs and MB produce both high PEC currents under 430 nm light and 627 nm light illumination,respectively,and the generated PEC currents are closely relied on target miRNAs amounts.Thus,highly sensitive and simultaneous detection of microRNA-21 and let-7a was readily achieved with detection limit at 6.6 fmol/L and 15.4 fmol/L based on 3σ,respectively.Further,this PEC biosensor was applied in simultaneous analysis of miRNA-21 and let-7a in breast cancer patient's serum with acceptable results.We expect this biosensor will find more useful application in diagnosis of miRNA-related diseases.