Abstract: This research measured the midgut protease enzyme activity in the red and green pea aphid (Acyrthosiphon pisum) after they had fed on 8 alfalfa (Medicago sativa) varieties with differing resistance. The midgut protease activity of the red pea aphid was (9.71±0.63) U/mL after they had fed on the variety Weinan, and was (0.33±0.10) U/mL after feeding on variety Sandili. The protease activity of the green pea aphid was (9.88±0.56) U/mL after feeding on the variety Hunter river, and (0.43±0.02) U/mL after feeding on Sandili. The midgut lipase activity of the red pea aphid was (4.12±0.30) U/mL when they infested the cultivar Hunter river, but was only (1.63±0.10) U/mL (P<0.05) when they infested the variety Gannong No.5. Lipase activity was (4.00±0.29) U/mL when aphids had fed on the variety Weinan, and was (1.08±0.13) U/mL when feeding on the cultivar Gannong No.5. The midgut amylase activities of the red and green pea aphids were (2.30±0.40) U/mL and (2.14±0.29) U/mL, respectively when they had fed on the cultivar Hunter river. In summary, for pea aphid infestation of 8 alfalfa varieties with differing aphid resistance, a high negative linear correlation was observed between the resistance of the different alfalfa varieties and the protease, lipase and amylase activities of the pea aphid, i.e. when the alfalfa aphid resistance is higher, the enzyme activity is lower. Hence it is concluded that the resistant alfalfa varieties restrict the midgut digestive enzyme activity of the pea aphid.