To improve vanadium leaching and yield rates during wet vanadium extraction processes, the crystallization behavior and growth mechanism of spinel crystals in vanadium-containing slags were investigated by the mathematical models and high temperature experiments. The results show that the suitable temperature range for spinel crystallization was 1473–1573 K. And the spinels’ crystallization abilities were ranked FeCr₂O₄ → FeV₂O₄ → Fe₂TiO₄. Although the concentration of chromium in the vanadium-containing slags was relatively low, the crystallization rate of all spinel crystals was greatly affected by the chromium spinels. The distribution of spinel diameters differed under the actual smelting condition, with the most common diameter range being 10–15 µm. In addition, the mean diameter and volume fraction of spinels increased with prolonging holding time and decreasing temperature in the actual smelting temperature range. Spinel crystals grew rapidly with time at 1623 K, the mean crystal diameter changed from 15.0 µm at 0 min to 24.9 µm at 60 min. And the mean diameter and volume fraction of spinels treated at 1523 K for 30 min were 28.1 µm and 46.3%, respectively. The mean diameter and volume fraction of spinel crystals increased with prolonging holding time and by decreasing the temperature in the actual smelting temperature range (1573–1673 K). Thus, the remaining slag process is beneficial to spinels growth in industrial vanadium-extraction process.