The invention discloses a CRISPR-Cas system for diagnosing spinal muscular atrophy. The system comprises Cas 12. Preferably, the CRISPR-Cas system includes Cas12, crRNA, dsDNA, and reporter RNA strands. At that same time, the invention also disclose application of the CRISPR-Cas system in that preparation of kit for diagnosis of spinal muscular atrophy. The CRISPR-CAS system can quickly detect themutation of SMN1 gene, the pathogenic gene of spinal muscular atrophy, and has the advantages of high specificity and sensitivity, intuitive analysis of results, accurate rate as high as 100%, etc. The CRISPR-Cas system is very suitable for large-scale clinical sample detection.