In heavy oil reservoirs, steam channeling and steam override seriously decrease oil production and the ultimate oil recovery during steam flooding. Aiming at the two problems, some experiments were carried out to analyze the EOR mechanisms through injecting foaming agents along with steam injection in heavy oil reservoirs. An orthogonal method was employed to analyze the multiple factors on foam's properties to optimize foaming agent for steam injection. Then a novel 2D-visualization experiment was carried out to quantitatively study the characteristics of steam channeling and the variation of sweep efficiency during steam or steam foams flooding. Based on the experimental results, many bubble's characteristics, such as migration, retention, regeneration and etc., were analyzed through the macroscopic and microscopic perspectives. The experimental results show that the Jamin effect increases the flow resistance of steam-phase in porous media to obviously enlarge the macro sweep efficiency and effectively increase micro oil displacement efficiency. On a macroscopic level, because of the unique structure, foams decrease steam override or steam channeling to improve sweep efficiency; on a microscopic level, due to the expansion effect of gas-phase, bubbles can desquamate the oil film on the pore wall and even the oil drop in the blind pore to decrease the residual oil saturation. In our experiments, the ultimate recovery of steam flooding can only reach 48.48%. However, the ultimate recovery of steam foams can reach 59.95%, which is 11.47% higher than steam flooding.