Abstract: In order to study the polymorphism of exon 4 of DQA2 gene in Holstein cattle and its correlation with lactation traits, PCR-SSCP method was used to detect the polymorphism of exon 4 of DQA2 gene in Holstein cattle, and further the relationship between exon 4 DQA2 gene and lactation traits was analyzed. The results showed that 4 polymorphic locus, three alleles（A,B,C） and five genotypes（AA,BB,CC,BC,AB） were detected, allele A and genotype AA was dominant. The results of population genetic parameters analysis showed homozygosty of exon 4 of DQA2 gene in Holstein cattle was high, polymorphism was abundant, and which deviated from Hardy-Weinberg equilibrium significantly（P＜0.01）. Correlation analysis showed that genotype BB of exon 4 of DQA2 gene in Holstein cattle had significant influence on somatic cell score（P＜0.05）, and also had certain influence on milk fat rate and milk protein rate, DQA2 gene might be a genetic marker for improving mastitis and lactation traits of Holstein cattle. The results of this study provide theoretical basis for screening molecular genetic markers for Holstein cattle.