Based on the current development of the gas chromatographic analysis technology and the status quo of the automatic chromatographic analysis of gas dissolved in transformer's insulating oil， a mathematical model for the lowest concentration of gas components in the gas phase is proposed， which can be detected by gas chromatography. By using the mathematical model and conforming with the lowest detectable concentration specification of gas components in oil and the sensitivity limit to the chromatographic analysis system， the minimum volume of the oil chamber and gas chamber can be calculated and designed. Based on this， a novel small vacuum on-line extraction system（10 cm×10 cm×20 cm） and a novel automatic instrument（5 cm×5 cm×2 cm） for the sample gases transmission is designed. The volume（0.769 mL） of the quantity fixing vessel is determined by experiment on standard gases and the whole system is examined by experiment on the sample gases. The result shows that the small vacuum extraction system can be satisfactorily used for the on-line application of the chromatographic analysis of gas dissolved in transformer insulating oil.