Objective：To construct a high sensitive electrochemical biosensor for the detection of microRNA（miRNA） based on the strand displacement amplification（SDA） and catalytic hairpin assembly（CHA） cascading amplification strategy. Methods：Target miR-21 initiated SDA to obtain mass target-related DNA which could further trigger CHA to obtain mass biotin-labeled dsDNA. And then，the obtained biotin-labeled dsDNA was immobilized onto the surface of the electrode modified with auxiliary probe. Later，ST-AP was modified onto the electrode surface by the specific recognition of biotin and streptavidin. When the α-NPP was in the test buffer，α-NPP was catalyzed by ST-AP to in situ produce α-NP on the surface of the electrode. Finally，the electrochemical signal of α-NP was correlated with the concentration of miR-21. Results：Under the optimized experimental conditions，the prepared biosensor showed a linear response to miR-21 range from 0.1 pmol/L to 10 nmol/L with the limit of detection low to 60 fmol/L. Conclusion：The designed biosensor has been exhibited high sensitivity and excellent specificity，which may provide a new idea and new platform for clinical determination of miR-21.