Abstract: The way of supercritical CO₂ fluid extraction was used in oil peony, and the effects of different conditions of different kinds of cosolvents and dosage on the yield of extraction of oil peony were studied. GC-MS technology was used for analysis of fatty acids composition in the oil peony. The result shows that at the extraction condition: 35 MPa for pressure, 25 g/min for CO₂ flow rate, 2 h for extraction time and 40 ℃ for temperature, the oil yield with shell has been increased 16.82% after using ethyl acetate as cosolvent. As well the oil yield with shell has been increased 6.55% after using petroleum ether as cosolvent. And the oil yield with shell has been increased 12.89% after using n-hexane as cosolvent. As can be seen from the results the oil yields with shell all have improved after using of cosolvents, and ethyl acetate has shown the optimum efficiency in oil yields in supercritical CO₂ fluid extraction. Comprehensively studied the dosages of 3 cosolvents, the optimal dosage of cosolvents was 10% of CO₂ flow rate of ethyl acetate. Peony oil fatty acid composition analysis showed that, oil peony riched in unsaturated fatty acid, especially α-linolenic acid, which take up 78.95% content of the total fatty acid.