Abstract: Steam-assisted gravity drainage (SAGD) is a mature thermal recovery technology for heavy oil. However, in the later stage of exploitation, as the contact area between steam chamber and cap rock further increases, the heat dissipation is increased, the oil displacement efficiency is decreased, and the cumulative oil-steam ratio declines. This paper investigated the replacement technology for injecting light hydrocarbon solvent in the mid-late period of SAGD. Using dimethyl ether as the auxiliary solvent and propane as the control, three sets of two-dimensional (2-D) visualization experiments were carried out to monitor the oil production, liquid production rate, water cut of produced liquid, crude oil recovery, cumulative oil-steam ratio and solvent recovery, and also discover the changes of temperature field and steam chamber. The experimental results show that SAGD recovery is mainly divided into three stages:upward development of steam chamber, lateral expansion of steam chamber, and downward development of steam chamber. When the steam chamber has extended to the boundary of cap rock, i.e., in the mid-late stage of SAGD production, the oil production rate is decreased significantly. After adding solvent in the mid-late stage of SAGD, oil production and liquid production rate are increased significantly. Compared with the oil recovery of 53.67 % and accumulative oil-steam ratio of 0.141 in SAGD process, propane- and dimethyl ether- assisted SAGD can increase the oil recovery by 11.91 % and 19.26 %, as well as the accumulative oil-steam ratio by 0.012 and 0.027, respectively. The recovery of dimethyl ether is 86.6 %, which is 6.9 % higher than that of propane.