The dynamic evolution of metallurgical slag foaming is vital for the efficiency and safety of the metallurgical processes. Based on the similarity principle via Morton number, the dynamic evolution of Basic Oxygen Furnace (BOF) slag foaming was studied through physical modelling in this investigation. The steady-state structure characteristics and the dynamic evolution of slag foaming were analyzed under the different of superficial gas velocity (us). It was found that the foaming height increased first and consequently decreased as the us increased, while the four different types of steady-state foam structure characteristics were summarized and the dynamic evolution of slag foaming was analyzed. At 0.08 mm∙s−1 < us ≤ 0.50 mm∙s−1, the foam system presented two-layer structure (called Partial Foaming I), while the foaming height increased linearly with the us increased; following gas supply interruption, the foam decayed at a uniform rate. At 0.50 mm∙s−1 < us < 10.00 mm∙s−1, the foam structure evolved into the Entire Foaming I, which was stacked by the spherical bubbles and the deformed bubbles; also, the decaying rate of the foam decreased as the liquid level decreased following the gas supply stop. When us was 0.60 mm∙s−1, the foam had the maximum stability, because its structure was in close proximity to densest packing of spherical bubbles. At 10.00 mm∙s−1 ≤ us ≤ 16.67 mm∙s−1, the foam structure of Entire Foaming II occurred, while the liquid level fluctuated sharply due to the poor stability of the polyhedral bubbles with high size at the upper levels. Moreover, the foaming height reached the peak value; in addition, the liquid level rapidly decreased due to the polyhedral bubble breakage after gas supply interruption. At 16.67 mm∙s−1 < us < 53.33 mm∙s−1, the foam structure evolved into the Partial Foaming II, due to the strong disturbance of high-speed air flow, while the foaming height decreased with the rise of us. As the us further increased, the high-speed gas flow penetrated the liquid phase, leading to the foam disappearance.