Existing CRISPR/cas-based biosensors usually improve sensitivity by target amplification, which is time-consuming and susceptible to impurities in complex biofluid. Herein, this is the first time a cascade CRISPR/cas (casCRISPR) system has been developed, which can provide a detection limit of 1.33 fM (∼1000 times lower than direct Cas13a-based miRNA detection) and single-base resolution for miR-17 detection without resorting to target amplification. casCRISPR can also be applied to detect miRNA in complicated cell extracts and serum samples. Overall, casCRISPR will provide a heuristic idea for CRISPR/cas based biosensing, and could be a promising tool for miRNA diagnostics.