The invention discloses a purpose of tobacco extracts for a citrus fruit fly insecticide, and belongs to the technical field of farm insecticide. The invention also provides the citrus fruit fly insecticide and an insect killing method. The insecticide contains 0.8 to 1.4mg/ml of tobacco extracts; the tobacco extracts are used for replacing chemical farm insecticide to prevent and control citrus fruit flies; the prevention effect is high; no farm insecticide residue exists; the target bioselectivity is high; the disease and pest medicine resistance is low, and the like. Meanwhile, a novel pathis provided for the reasonable development and application of waste and abandoned tobacco leaves.