Abstract: The paper uses the 2 indexes of accumulative oil increment and oil exchange ratio to evaluate the field test effects of CO2 huff and puff in 7 typical horizontal wells in G block of Jidong oilfield, performs the production history matching through numerical simulation and studies the mechanism of CO2 huff and puff EOR and its main influence factors by combining with geological and engineering parameters. The study results show that the higher the oil saturation around horizontal wells, the better the EOR effects by CO2 huff and puff in horizontal wells; the longer the horizontal section of horizontal wells, the longer the perforated intervals, the larger the laterally swept areas of CO2, and the less of CO2 consumption in bottom-water layer, the better huff and puff effect will be gained. During CO2 huff and puff, CO2 is not only dissolved in the oil, but also can drive the oil away from the well bottom under the injection pressure, which could result in the effects of adjacent wells. After CO2 huff and puff, the oil saturation around the horizontal well bottom drops dramatically and a cavity will form, which can influence the effect of next cycle of CO2 huff and puff