Abstract: Supercritical steam technique can take the oilfield produced water as the fuel and its generated supercritical steam has higher enthalpy, so its application prospect to develop offshore extra heavy oil is promising. So far, however, the stimulation mechanisms of developing extra heavy oil by supercritical steam hasn’t been clarified. The development process of heavy oil by supercritical steam was simulated with the LD5-2N typical extra heavy oil reservoir of the Bohai Oilfield as the example, by using the newly established experimental device of supercritical steam flooding. Then, the stimulation mechanisms of developing extra heavy oil by supercritical steam were studied and the change laws of the physical properties of crude oil and reservoir under supercritical condition were analyzed. It is indicated that the supercritical steam technique can enhance the recovery factor greatly and its stimulation mechanisms are embodied as higher enthalpy to reduce oil viscosity significantly, high solubility to produce the similar effects to miscible flooding and high diffusibility to move into the micropore structures in cores more easily. The oil displacement efficiency of supercritical steam flooding can reach 94%. Under the supercritical condition, asphaltenic and colloidal compositions in extra heavy oil undergo thermal cracking reaction, which decreases the oil viscosity by 30% further. Rock dissolution can happen, which leads to the increase of reservoir porosity and permeability. And the maximum increase amplitude of the permeability is up to 21.7%. The research results provide the theoretical basis for the development of offshore extra heavy oil reservoirs by supercritical steam.