Gene knockout method based on base editing and its application is provided. The gene knockout method comprises: selecting a 20 bp-NGG target sequence of the coding region of the gene to be knocked out, so that it contains a complete target codon CAA, CAG or CGA; and using sgRNA sequence to locate BE3 to the target sequence, to convert the target single-base C of the target codon into T and thus introduce a corresponding termination codon TAA, TAG orTGA in order to realize the knockout, wherein the target single-base C is located preferably on site 4-8 in the target sequence, the interval between the target codon and NGG is 12 to 14 bp, and the upstream base (H) near the target codon cannot be G; and the sgRNA sequence is a 20 bp sequence complementary to the target sequence.