Abstract: To explore the function of carbonic anhydrase 12 (ca12) in grass carp (Ctenopharyngodon idella) response to carbonate alkalinity stress, we subjected grass carp to alkalinity stress at different NaHCO₃ concentration gradients (0, 20, 40, 60, 80, 100, 150 mmol/L), and systematically analyzed the expression level of ca12 in various tissues (brain, heart, kidney, and gill) as well as the histopathological changes in gill and intestinal tissues. Compared with wild-type grass carp, chimeric grass carp edited by CRISPR/Cas9 technology had higher survival rate at different alkalinity levels and time points. Moreover, the expression levels of ca12 in the kidney and gill tissues were significantly downregulated with increasing alkalinity. Under stimulation with 60 mM/L NaHCO₃, gill and intestinal tissues of chimeric grass carp showed adaptive morphology changes, with a significantly lower apoptosis rate than that of wild-type grass carp, and the survival rate of chimeric grass carp was also higher under this condition. The study indicates that high alkalinity stress exerts adverse effects on grass carp, and the chimeric grass carp edited by CRISPR/Cas9 technology display significantly higher survival rates in high-alkalinity water than wild-type grass carp. This suggests that the ca12 plays an important regulatory role in grass carp under high-alkaline conditions, making it a critical target for grass carp adaption to high-alkaline environments. Loss of ca12 function significantly enhances the alkalinity tolerance of grass carp, providing a new strategy to improve the aquaculture rate in saline-alkali areas and expand the development space of the fishery industry.