Background: Farmers have applied Cynanchum mongolicum (Maxim) to control crop pests. The aim of this study was to analyze composition of essential oil from C. mongolicum, evaluate insecticidal activities against Aphis glycines, and lethal doses. Materials and Methods: Essential oil from C. mongolicum was efficiently extracted by steam distillation. The main components of the oil were analyzed with a gas chromatography/mass spectrometry (GC/MS) system, and the insecticidal activity of the essential oil on soybean aphids Aphis glycines was tested using a variety of methods. Results: The components of the essential oil from C. mongolicum mainly included terpenes and ester compounds, of which (Z)-3-Hexen-1-ol acetate, cis-3-hexenyl isovalerate, α-farnesene, and β-caryophyllene accounted for 15.8, 10.4, 8.4, and 5.5%, respectively. With 1- and 2-day exposure, the essential oil showed pronounced contact toxicity (median lethal concentration (LC 50 ) =37.8 and 38.4 μL/mL, respectively), weak fumigant toxicity (LC 50 = 139.7 and 139.9 μL/L, respectively). The essential oil showed strong deterrent activity on soybean aphids in 2 and 4 h. Conclusion: The essential oil of C. mongolicum contained insecticidal components and possessed contact toxicity and deterrent activity to A. glycines.