The invention relates to the technical field of yarn dyeing and finishing, and particularly discloses a polyester-cotton blended yarn dyeing process. The polyester-cotton blended yarn dyeing process comprises the following steps of S1, performing yarn pretreatment; S2, performing washing and drying; S3, performing primary dyeing: adding 1.5-3 g / L of a disperse dye leveling agent into a primary dyeing bathtub, performing heating to 75 DEG C, adding a disperse dye, then performing heating to 85 DEG C, sending the washed and dried yarns into primary dyeing liquid for dip dyeing at a bath ratio of1: (9-11), and performing heat preservation dyeing for 45-55min; S4, performing reduction cleaning; S5, performing secondary dyeing, wherein the bath ratio is 1: (9-11), the temperature is increasedto 75-85 DEG C, heat preservation is performed for 18-22 min, then the temperature is increased to 90 DEG C, heat preservation dyeing is performed for 30-35 min, and secondary dyeing liquid is activedyeing liquid; S6, performing soaping; and S7, performing dehydration and drying, wherein pretreatment liquid comprises the following components in parts by weight: 3-6 parts of pectinase, 4-8 parts of alpha-amylase, 4-7 parts of hydrogen peroxide, 0.5-1 part of sodium silicate, 1-3 parts of a penetrant, 0.5-1 part of cocamidopropyl hydroxy sulfobetaine and 65-85 parts of deionized water. The method has the advantages of effectively improving the dyeing uniformity of the cotton-polyester blended yarns and improving the yarn quality.