The development of a sensitive, facile, and cost-effective colorimetric method is of great significance for the point-of-care testing of viral nucleic acid. Herein, we reported a strand displacement amplification assisted CRISPR-Cas12a (SDACC) method for the colorimetric analysis of viral nucleic acid. The hepatitis B virus (HBV) DNA was chosen as the target to trigger strand displacement amplification (SDA) and generate abundant single-strand DNA (ssDNA) products. The ssDNA amplicon hybridized with template DNA to activate the trans-cleavage activity of CRISPR-Cas12a, leading to the nonspecific cleavage of ssDNA on GOx-ssDNA-modified magnetic beads and the release of GOx. The released GOx was capable of catalyzing the substrate solution to generate a color change, which could be directly observed by naked eyes. The SDACC strategy could identify a single-base mismatch located in the DNA sequence and achieve a sensitive detection for HBV DNA with the limit of detection as low as 41.8 fM. Notably, the sophisticated primer design for target amplification and complicated detection process could be circumvented. The current approach realizes a simple, low-cost, and sensitive colorimetric detection for viral nucleic acid and holds great promise for the practical application of virus infection diagnosis.