To investigate the effect of ABO gene α-1,3-D galactosyl transferase mutation on B antigen expression and its molecular mechanism.The proband and their family members were identified by routine serological methods, and ABO genotyping and sequence analysis were performed by polymerase chain reaction-sequence specificity (PCR-SSP) and direct sequencing of PCR products from exon 1-7 of ABO gene. The 3D structural simulation of mutant proteins was performed by bioinformatics software. The effect of gene mutation on protein structural stability was analyzed.The proband and his family members were subtype B. ABO genotyping indicated that the proband's genotype was Bw12/O. Gene sequencing results confirmed the presence of ABO*BW.12 characteristic variation c.278C>T in the 6th exon of allele B, leading to the replacement of polypeptide chain p.Pro93Leu. The 3D structure simulation analysis of the protein showed that the hydrogen bonds and water molecules connected to the protein changed after amino acid substitution. The family investigation found that the grandfather, father, uncle and brother of the proband all carried the same ABO*BW.12 allele.The mutation of the 6th exon c.278C>T of ABO gene led to the substitution of polypeptide chain amino acids, which affected the stability of α-1,3-D galactosyl transferase protein, resulting in the change of enzyme activity, and the Bw.12 phenotype, which can be stably inherited.