In this work, we demonstrate for the first time the application of the phosphorothioated-terminal hairpin formation and self-priming extension (PS-THSP) reaction for miRNA assays. A self-priming amplification accelerating CRISPR sensor was well-established for sensitive and specific miRNA detection by integrating the PS-THSP reaction and CRISPR/Cas12a system. The sensor consists of three steps: (1) the formation of a complete PS-THSP template in the presence of target miRNA and ligase; (2) the exponential isothermal amplification of the PS-THSP reaction under the action of DNA polymerase; (3) the activation of the CRISPR/Cas12a fluorescence system to generate signals. We used miR-21 as a model target. The sensor can achieve sensitive detection of miR-21 without the involvement of any primers, and the special design of the CRISPR proto-spacer neighbor motif (PAM) sequence effectively avoids the interference of the background signal. In addition, the sensor can not only identify single-base mutant homologous sequences but also show stable performance in complex biological matrices. We have successfully used this sensor to accurately analyze miR-21 in different cell lines and real clinical samples, demonstrating its great potential in clinical diagnosis.