SAGD is an effective technology that was devised for the development of oil sand. However, nowadays, most research is focused on the two-dimensional (2D) vertical profile, which ignores the non uniformity of steam along the horizontal well occurring in an oilfield. In this paper, based on the properties in MacKay River oil sands, an experimental device for SAGD is well-designed to investigate steam-chamber expansion along the horizontal well. Then, based on experimental results, a new 3D mathematical model considering steam non-uniformity was established to predict production performance and steam-chamber expansion. Next, the calculated production performance and steam chamber evolution were compared with numerical data to verify the accuracy of the model. Finally, the model was compared with previous models to assess its quality. Results show the following. (a) The steam distribution along the horizontal direction is non-uniform; even the preheating of SAGD is uniform using the modified pre-heating method, which, when switched to formal SAGD production, also presents the same non-uniform steam distribution. (b) The comparisons reveal that the proposed 3D model is reasonably more accurate than previous models in predicting cumulative oil prediction and steam-chamber expansion. (c) Factors influencing steam distribution along the horizontal well are also discussed, and it is believed that steam quality is the deciding factor that determines the degree of steam non-uniformity. (C) 2018 Elsevier Ltd. All rights reserved.