The thermodynamic equilibrium of deoxidation reactions between molten slag and steel was calculated using a slag-steel coupling thermodynamic model and the mass conservation model based on the ionmolecular coexistence theory. The study focused on the effects of slag composition and deoxidizer type on the oxygen content of low alloy steel during the electroslag remelting (ESR) process. The measured and predicted values of the oxygen content in remelted ingots, and the contents of FeO and MnO in slags were compared and analyzed. Results show that the measured content of total oxygen has a certain correlation with the trend of dissolved oxygen predicted by the model when using Ca-Si alloys as deoxidizer, but it is not correlated with the trend of dissolved oxygen predicted by the model when using Al as deoxidizer. The deoxidation mechanisms of Ca-Si and Al are different. Ca-Si alloy directly reacts with FeO and MnO in slag to reduce the oxygen potential of slag, hence it can inhibit the transfer of oxygen from the slag to molten steel. While, when Al deoxidizer is used, the oxygen content in steel is mainly reduced through floating up the alumina inclusions. Compared to Al, utilizing Ca-Si alloy as a deoxidizer is more effective in reducing the oxygen content and the amount of inclusions in ESR ingot.