Abstract: Based on a developed physical model during the mixing process in the tundish and the liquid pool of the strand, intermixed length and composition variation model have been established during steel grade changes in the continuous slab casting processes. The research object was the mixing process in the single-strand slab caster during steel grade changes of Q235 and Q335Ti steel with 220 mm × 1560 mm section. Key parameters of the model were determined using the water model test and numerical simulation, and the model was verified through plant tests, which were performed on the slab caster during the grade transition period of continuous casting. Real grade intermixed slabs were produced, and composition distributions were measured and compared. The carbon composition and the length of the intermixed slab predicted using the model were found to be in good agreement with the experimental data. Mixed behaviors in the tundish and strand were tracked using the model under various mixing conditions. In addition, the length and the composition change law of the intermixed slab can be precisely predicted. The effect of casting speed and the remaining molten steel in the tundish on length and the composition change law of the intermixed slab were studied by the model. As the casting speed remains unchanged, the intermixed-slab length increases with more remaining molten steel in the tundish. While the mass of remaining molten steel in the tundish keeps unchanged, the intermixed slab length decreases with more casting speed. In comparison, the remaining molten steel in the tundish has a larger effect on the intermixed slab length than the casting speed. As the mass of the remaining molten steel in the tundish increases with constant casting speed, the rate at which the mass fraction of C changes from 0.16% to 0.18% slows down; While the mass of remaining molten steel in the tundish keeps unchanged, the rate at which the mass fraction of C changes from 0.16% to 0.18% accelerates with an increase in the casting speed. Reducing the mass of the residual molten steel in the tundish and increasing the casting speed in the strand is beneficial for the reduction in the length of the intermixed slab and element composition variation rate. Moreover, the strategy of lowering the liquid level in the tundish and increasing the casting speed simultaneously can be adopted to decrease the intermixed slab length to the greatest extent.