At present, offshore heavy oil thermal recovery mainly uses oil-fired boilers to generate high-dryness steam. The flue gas generated after boiler combustion contains CO2 and other components. The flue gas is treated and reinjected for use, which can not only improve oil recovery, but also reduce carbon emissions. The article carried out the crude oil combustion experiment, explored the content of carbon dioxide, sulfur dioxide, nitrogen oxide and dust in the combustion products, and innovated and developed the process and equipment applicable to the flue gas treatment and reinjection of oil-fired boilers in offshore oil fields. After the flue gas treatment, it was injected into the heavy oil wells with steam. The experiment showed that the injection of steam and gas at the same time could increase the periodic oil production by 2 to 3 times compared with the injection of steam only, and the synergy between the two was significant. Relevant equipment has been installed on a heavy oil thermal recovery development platform in Bohai Oilfield, which will provide technical reference for the exploration of effective ways to improve quality and efficiency of large-scale thermal recovery of offshore heavy oil and energy conservation and environmental protection of thermal injection equipment.