In order to study the effect of CO2 flooding on the physical properties of crude oil, understand the mechanism of CO2 flooding, and provide theoretical basis for the analysis of field test results of CO2 flooding, the field test of CO2 flooding in Huang 3 block of Jiyuan oilfield in Changqing was carried out by column chromatography and gas chromatography-mass spectrometry. The physical properties of crude oil produced before and after CO2 flooding in Huang 3 block of Jiyuan oilfield, Changqing, were studied in detail by means of column chromatography, gas chromatography and mass spectrometry method and other methods. The changes of crude oil group content, different carbon chain alkanes content, solidification point and viscosity-temperature curve were mainly investigated. The experimental results show that after CO2 injection, the alkane fraction of crude oil first decreases and then increases, while the non-hydrocarbon and asphaltene fraction first increases and then decreases; the light alkane（C8—C16） in the alkane fraction of crude oil first decreases and then increases, the heavy alkane（C17—C40） content first increases and then decreases; the pour point of crude oil first increases and then decreases, and the viscosity of crude oil at low temperature（35 ℃） increased slightly, but there was no obvious change at high temperature. The above results accord with the mechanism of CO2 miscible flooding. The change of crude oil physical properties is the result of the interaction of CO2 extraction mechanism. Through the above research, it provides data support for understanding the mechanism of CO2 flooding, and provides a certain basis for further formulating plans to improve the efficiency of CO2 flooding.