The volatile oils of Radix Bupleuri extracted under decompression and atmospheric pressure were compared in terms of composition,content and efficacy to investigate the quality of the volatile oil and optimize the extraction method.Gas chromatography-mass spectrometry(GC-MS) was used to analyze the constituents of volatile oils and their relative contents.Meanwhile,a method of gas chromatography(GC) was established for the determination of hexanal,heptanal,octanal and trans 2,4 decadienal in the volatile oils,and the contents of active components in volatile oil extracted under different pressures were determined.A rat fever model was reproduced by subcutaneous injection of yeast suspension on the back,the volatile oils extracted from Radix Bupleuri under atmospheric pressure and decompression were dissolved in 1% Tween 80 and injected intraperitoneally.The temperature changes of rats at different time points were measured,and single factor analysis of variance was used for intergroup comparison.The composition and content of the volatile oil extracted under decompression conditions were different from those extracted under atmospheric pressure,the extraction rates of hexanal,heptanal,octanal and trans 2,4 decadienal were higher,and the effect of the volatile oil on the fever caused by dry yeast was more significant.Results showed that decompression could effectively reduce the damage of heat sensitive components and improve the extraction efficiency of effective components.The volatile oil of Radix Bupleuri extracted by this method has a more significant medicinal effect,and its quality is better than that extracted under atmospheric pressure.