In order to research the enhanced oil recovery mechanism and influence factors of CO₂ puff and huff in horizontal well in heavy oil reservoir with bottom water，the CO₂ swell experiments was carried out and CO₂ puff and huff single well simulation model was built in view of G104-5 reservoir of M oilfield. The phase changing after CO₂ swell，and the influence of technological parameters of gas injection and reservoir parameters were studied，and the CO₂ puff and huff of 7 horizontal wells from G104-5 was conducted. It is concluded that after injected 35mol% CO₂，the viscosity of heavy oil decreased by 45% and the coefficient of cubic expansion was 1.13，and saturation pressure increased by 200% and mole fraction of C₉—C₂₂ in oil decreased by 14.4mol%. After CO₂ puff and huff in the thin bottom water reservoir，the water cut of vertical well（shoot layer from 1 to 3），which had 313.17 t cumulative incremental oil，decreased to 87%，while the water cut of the horizon well，the cumulative incremental oil being of 679.91 t，decreased to 75%，which indicted that the latter had better oil-increment and water-control effect. Among all of the effect parameters，the gas volume，reservoir thickness，oil saturation，porosity was the most important parameters；while the injection speed，formation pressure，shut time，exploit speed and permeability had little effect in CO₂ huff and puff with horizon well. In the 7 CO₂ huff and puff wells of G104-5 reservoir，G104-5P101 had the biggest porosity（45%），the maximum injected gas volume（412 t），and the most cumulative incremental oil（1412.3 t）.