Based on the water model experiment of an iron bath smelting reduction reactor with thick slag layer, numerical simulation is taken out to study the stirring characteristics of the molten bath by double-layer side-blowing lance under different injection angles (40°, 45°, 50°) and different horizontal insertion depths (60, 120, 180 mm), and the accuracy of numerical simulation is verified by the results of water model experiment. By observing the flow field of the molten pool in the gas-liquid two-phase flow and the diffusion of the tracer, the proportion of the weak flow zone and the dead zone and the mixing time under different side-blowing conditions are analyzed, the results show that: the horizontal insertion angle of the side-blowing lance has a more significant influence on the stirring characteristics of the molten pool than the horizontal insertion depth. The horizontal injection angle and insertion depth of the side-blowing lance affect the swirling flow area and the number of swirling flows generated in the molten pool. When the horizontal injection angle of the side-blowing lance is larger and the horizontal insertion depth is deeper, the volume of the weak flow zone or the dead zone in the molten pool is smaller, which is more conducive to the stirring and mixing of the molten pool. The best injection conditions are obtained from the numerical simulation: the upper injection angle is 50°, the insertion depth is 60 mm, the lower injection angle is 50°, and the insertion depth is 120 mm. In addition, the mixing time of molten bath under different gas flow rates are also studied. It is obtained that the mixing time of molten bath is the shortest when the single gas flow rates of upper and lower lance are 8.58 and 10.43 Nm3/h, respectively.