Adding chemical additives with in-situ generation of foam is an approach to enhance SAGD (steam assisted gravity drainage) performance both in terms of oil production and SOR (steam oil ratio). Simulation study tells that, owing to gas mobility control, interfacial tension reduction and emulsification, the steam chamber profile is substantially controlled with a reduced heat loss, and the residual oil saturation drops dramatically. A heterogeneous model based on a Suncor Firebag project is further employed to testify that bubbles are conducive to improve volumetric sweep efficiency by diverting steam into low-permeable area. Simultaneously, foam favors to reduce the influences of top water zone and maintain a bowl-shaped and uniformly-developed steam chamber growth. Afterwards, an analytical method is introduced to further explain the physical mechanisms with a modified finger rising model, which shows that CAFA-SAGD (chemical additives and foam assisted SAGD) owns a lower finger rising velocity with less steam consumption.