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DNAzyme amplifiers have been extensively explored as a useful sensing platform, but single DNAzyme amplifier is limited in biosensing applications by its low sensitivity. Herein, a cascade DNAzyme amplifier was designed by exploiting concurrent amplification cycle principles of toehold-mediated strand displacement reaction (TSDR) and Zn2+-assisted DNAzyme cycle with lower cost and simpler procedures. Compared with single DNAzyme amplifier, the proposed TSDR-propelled cascade DNAzyme amplifier exhibited higher sensitivity by releasing more DNAzyme through TSDR to cleave substrate strand during the DNAzyme cycle. Base on this, let-7a could be sensitively detected in the range of 5–50 nmol/L with a detection limit of 64 pmol/L. Furthermore, the dual signal amplification strategy of the cascade DNAzyme amplifier exhibited excellent selectivity to distinguish single-base mismatched DNA strands, which has been successfully applied to the determination of let-7a in blood serum, showing high promise in early cancer diagnosis.