Abstract: Hypecoum leptocarpum was used as a source of extracts to test insecticidal activity against Mythimna seperata. Two extraction methods (constant temperature oscillation method and Soxhlet extraction method) and four solvents (ethanol, methanol, hexane, dichloromethane) were used to study the mode of action of eight kinds of extracts on M. seperata. Ethanol and Soxhlet extraction method were the best insecticidal solvent and extraction method respectively. The eight kinds of extracts of H. leptocarpum showed strong antifeedant and growth inhibition effects on the 3 instar larvae of M. seperata and after 24 and 48 h feeding, the inhibition rates were above 70%, and with the ethanol extract were up to 90.78%. The four kinds of solvent extracts inhibited growth and development of M. seperata. The inhibition rates at 24 h after treatment were 74.1%-91.45%, and the ethanol extracts at 48 h after treatment was 87.00% with a final pupation rate of 23.33%. The growth rate was slow, and the progress of growth and development of M. seperata was delayed. The eight kinds of extracts had weak stomach and contact toxicity. The ethanol extract of 48 h contact and stomach poison was the highest, and the correction mortality was 33.33% and 23.30% respectively. The anti-feeding component of the H. leptocarpum extracts was mainly concentrated in the aboveground plant parts.