Response surface methodology was used to optimize the extraction conditions of onion oil from purple Allium cepa L.The yield of purple onion oil was the index,extraction solvent,extraction time,ratio of solid to solvent and reflux temperature were studied as factors by Soxhlet extraction method.Based on the single factor experiments,a three-factor-three-level experiment design has been developed by Box-Benhnken central composite design method.The response surface method was applied to analyze the results of experiments.The results showed that the optimum extracting conditions for purple onion oil were as follows:solvent was petroleum ether,the ratio of solid to solvent was 3.64/80 g∶mL,extraction time was 3.83h and reflux temperature was 78.49℃.Under these conditions the predictive maximum yield of purple onion oil was 0.462%,the actual yield was 0.470%.the relative error was 1.73%.