In order to improve the extraction rate of onion essential oil and make the extraction process of onion essential oil easy to operate, three drying methods, including hot air drying (AD), vacuum freeze drying (FD) and microwave freeze drying (MFD), were used to make fresh onion powder. The essential oil of onion was extracted by distillation. The extraction rate and the DPPH free radical scavenging ability of three kinds of dried onion powder were compared. The extraction rate of onion essential oil from onion powder by wave freeze-drying was 4.57%, with shortest extraction time. The optimum extraction conditions of onion essential oil were determined by orthogonal experiment, i.e. MFD onion powder, warm-extracted at 40 ℃ for 50 min. The onion essential oil obtained under the control of these conditions has strong DPPH radical scavenging capacity.