The high quality waste heat recovery of the molten blast furnace slag was necessary and urgent. In this paper, the flow characteristic of the molten slag reactor was investigated by numerical simulation. Firstly, the three-dimensional model was established for investigating the gas-liquid two-phase flow. The gas-liquid flow was modeled to be turbulent, which was described by the RNG k-epsilon model, and the interface of gas and liquid was conducted by the VOF model. Secondly, the top-submerged cold experiment system was constructed to validate the accuracy of the simulation model. Thirdly, the bubble behavior, gas phase distribution and molten slag motion were investigated. The bubble in the reactor would go through five stages and the maximum gas fraction reached about 4.87% at 0.75s. Meanwhile, the gas phase distribution and molten slag motion were closely related to the bubble behavior. Ultimately, the matrix analysis method was applied to obtain the optimal parameters of the reactor. The optimal condition improved the flow behavior in the molten BFS reactor significantly. The present results of the simulation provided an insight for the gas-liquid two-phase flow in molten slag reactor, which would provide the theoretical guidance for industrial applications. (C) 2019 Elsevier Ltd. All rights reserved.