The invention discloses a molecular marker method of a beef cattle fat color trait. The method comprises steps as follows: DNA extraction of bovine genome, design of a third exon primer of a BCO2 (beta-carotene oxygenase 2) gene, in-vitro amplification and genotype detection. The method can be used for assistant selection of the fat color trait in cattle breeding and can realize early selection of breeder cattle; with the adoption of the gene pyramiding method, the good genotype of the BCO2 gene can be stabilized after one generation, that is, homozygosis of a good gene of the fat color trait of a beef cattle strain is realized through selection of one generation, the generation interval is greatly shortened, and the selection process is accelerated. The molecular marker method is simple and convenient to operate, condition requirements in the polymerase chain reaction process are low, the length of amplified fragments is short (525bp), amplification is easier, and the amplification efficiency and the judgement accuracy of the genotype are improved.