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CRISPR-Cas12a based one-step assays are widely used for nucleic acid detection, particularly for pathogen detection. However, the detection capability of the one-step assay is reduced because the Cas12a protein competes with the isothermal amplification enzymes for the target DNA and cleaves it. Therefore, the key to improving the sensitivity of the one-step assay is to address the imbalance between isothermal amplification and CRISPR detection. In previous study, we developed a Cas12a one-step assay using single-stranded DNA (ssDNA)-modified crRNA (mD-crRNA) and applied this method for the detection of pathogenic DNA. Here, we utilized mD-crRNA to establish a sensitive one-step assay that enables the visual detection of SARS-CoV-2 under ultraviolet light, achieving a detection limit of 5 aM without cross-reactivity. The sensitivity of mD-crRNA in the one-step assay was 100-fold higher than that of wild-type crRNA. Mechanistic studies revealed that the addition of ssDNA at the 3′ end of mD-crRNA attenuates the binding affinity between the Cas12a-mD-crRNA complex and the target DNA. Consequently, this reduction in binding affinity decreases the cis -cleavage activity of Cas12a, mitigating its cleavage of the target DNA in the one-step assay. As a result, there is an augmentation in the amplification and accumulation of target DNA, thereby enhancing detection sensitivity. In the clinical testing of 40 SARS-CoV-2 RNA samples, the concordance between the results of the one-step assay and known qPCR results was 97.5 %. The one-step assay using mD-crRNA proves to be highly sensitive and specificity and visually effective for the detection of SARS-CoV-2. Our study delves into the application of the mD-crRNA-mediated one-step assay in nucleic acid detection and its associated reaction mechanism. This holds great significance in addressing the inherent incompatibility issues between isothermal amplification and CRISPR detection. [Display omitted] • The one-step assay using mD-crRNA for detecting the SARS-CoV-2 was established. • The 3′ end ssDNA in mD-crRNA was crucial for enhancing one-step assay sensitivity. • Reduced affinity of Cas12a-mD-crRNA and target DNA lowered Cas12a cleavage activity. • This assay demonstrated a visual detection limit of 5 aM for SARS-CoV-2. [ABSTRACT FROM AUTHOR]