_{Due to high oil viscosity, low permeability, thin reservoir formation, sand production, complex ground conditions, etc., Liu Guan-zhuang Chenglong 1601 Block in Dagang Oilfield, which is a heavy oil reservoir, cannot be developed effectively through conditional development technology. Therefore, horizontal well + CO2 huff and puff technology was proposed to solve this problem. And the experimental researches and the field test were conducted. Through laboratory experiments and numerical simulations, the mechanisms of enhanced oil recovery were studied and the injection-production parameters were optimized. The results showed that, after CO2 was dissolved in the heavy oil, the viscosity of the oil decreased from 16 800 mPa·s to 69 mPa·s, and the viscosity reduction rate was 99.6%; the threshold pressure gradient of the oil dropped from 19.1 MPa/m to 0.2 MPa/m. The absolute permeability increased by about 41.86%. Therefore, the heavy oil was more flowability. The optimal injected CO2 mass per unit length was between 3.5 and 5.5 t/m, the optimal CO2 injection rate per unit length was between 0.5 and 0.6 t/(m·d), the optimal shut in period was between 25 and 35 days, the optimal liquid production rate per day per unit length was between 0.075 and 0.125 m3 /(m·d). Two kinds of integrated injection and production string, namely, the combination of oil pumping unit, hydraulic feedback pump and electric heating rod, hydraulic jet pump were employed. The horizontal well + CO2 huff and puff technology was carried out in field test, and significant technical and economic effects were achieved, which proved the reliability and practicality of this technology. A single round of CO2 huff and puff process obtained an oil increase of 1000~2000 tons, with the maximum oil increase of more than 4,000 tons. The oil production per mass CO2 was over 1.72 t/t. This technology provides a good reference for the economic exploration and development in the same type of oil reservoir}