As a major technique for enhancing heavy-oil recovery, steam flooding has been used successfully at large scales around the world. Long-term indoor experiments and field trials, however, have revealed problems, such as heat loss to the overburden and a limited steam zone for steam flooding in thin heavy-oil reservoirs, which severely limit the thermal efficiency of the steam and the economic benefits of the reservoirs. Therefore, we established a set of two-dimensional sandpack models to study the steam chamber expansion characteristics of the horizontalwell solvent-enhanced steam flooding (HW-SESF) of different solvents and its influence on production performance. We established a numerical model based on the experimental parameters to simulate the solvent migration in the steam chamber and its effect on temperature, viscosity, and oil saturation profile. The results demonstrate that, compared with steam flooding, the HW-SESF strategy can effectively reduce the oil viscosity both at the front and inside the steam chamber and improve the steam-injection capability (the stronger the steam-injection capability is, the easier the steam can be injected into the model) of the horizontal injection well, which enhances displacement efficiency and enlarges the swept area of the steam chamber. For HW-SESF of light solvents, the displacement efficiency at the low water-cut production was high, and water cut increased rapidly as the steam front reached the production well. For HW-SESF of medium solvents, the period of stabilized production was long, and the recovery ratio was the highest compared with other solvents. For HW-SESF of heavy solvents, the peak oil production rate was high, and the oil production rate dropped quickly. Therefore, the HW-SESF strategy reduced the heavy-oil viscosity more effectively than conventional steam flooding, and thus the zone of the steam chamber was enlarged and the recovery ratio enhanced. Compared with light and heavy solvents, medium solvents can achieve the best results for steam chamber optimization and the highest recovery ratio; thus, it is considered the most effective method to exploit thin heavy-oil reservoirs.