Steam-assisted gravity drainage（SAGD）is a thermal recovery technology for the efficient development of heavy oil reservoirs. The Long Lake Oilfield in Canada is faced with problems such as high viscosity of crude oil，low permeability and large thickness of the sand-mud interbed，and difficult crude oil production above the interbed. In response，laboratory experiments on the start-up pressure gradient of heavy oil at different permeability and temperature were carried out. The feasibility of crude oil passing through the interbed under gravity was analyzed. On this basis，three sets of two-dimensional SAGD physical simulation experiments were carried out to study the influence of the thickness of the sand-mud interbed on the expansion of the SAGD steam chamber and the production effect of SAGD and obtain the expansion thickness limit of the SAGD steam chamber under the condition of the sand-mud interbed. The results show that the flow of heavy oil in the interbed has a start-up pressure gradient in the process of oil sand exploitation. The crude oil above the interbed will start the flow merely by gravity only when the temperature in the formation above the interbed reaches 200 ℃ or more and the viscosity of the crude oil decreases to below 8.966 mPa·s. The steam chamber shows a trend of the vertical expansion weakening，the horizontal expansion increasing，and the ability of the steam to break through the interbed declining as the thickness of the sand-mud interbeds in the formations increases. The steam can break through the interbed and the crude oil above the interbed can be produced to a certain extent when the permeability of the sand-mud interbed is 50 mD and its thickness is less than 6 cm.