The present study was motivated to reveal the oxidation mechanism of Al, Ti and Si elements during electroslag remelting (ESR) and propose methods to control them. Three small-scale ESR experiments were carried out with Incoloy 825 alloy as consumable electrode and three groups of slags with different TiO2 contents as raw materials. A kinetic model of slag-steel reaction was established based on ions and molecules coexistence theory (IMCT), film theory and penetration theory. The results show that Al and Si contents increase compared to that of the electrode but Ti content decreases. The increase of Al and the loss of Ti decrease with the increase of TiO2 in slag. The calculated values of the model are in good agreement with the experimental values. In the early stage of ESR process, FeO content increases apparently, so it is more effective to control oxidation in order to obtain uniform ingot. The restricted link of the reaction in the mass transfer of Al + Al2O3, Si + SiO2 and Ti + TiO2 is on the metal side, on the slag side and both sides, respectively. When TiO2 content in slag is lower than 12%, TiO2 addition is more effective. When TiO2 content is higher, adding Al deoxidizer is more effective to control Al and Ti contents.