The invention discloses an insecticide for killing potato beetles and a preparation method thereof. The insecticide is prepared from, by weight, 15-25 parts of azadirachtin, 10-15 parts of avermectin, 20-30 parts of carbosulfan, 0.5-1.5 parts of stabilizer, 0.05-0.1 part of active polypeptide, 1-5 parts of dispersant, 1-5 parts of emulsifier, 1-5 parts of auxiliary solvent, 2-8 parts of filler, and 20-30 parts of water. The insecticide for killing potato beetles has the advantages that the insecticide belongs to the pesticide compound with different insecticidal mechanisms, has more action point locations for the potato beetles, is quick in effect and long in duration, and can effectively control the potato beetles; the problem of pest resistance is effectively prevented, the problem of pest resistance at present is alleviated to the largest extent, so that the amount of pesticide used is reduced, the impact on the environment is also eased, and the insecticide is in line with the social green trend of environmental protection.