<p>Comparative analysis of inclusions modification by various calcium-magnesium composite treatments in AH36 ship plate steel were performed based on comprehensive thermodynamic calculations and high-temperature experiment. The results showed that Al₂O₃ or MgAl₂O₄ inclusions were formed under the casting temperature of AH36 ship plate steel. Through thermodynamic analysis, the content of inclusions produced by the first calcium and then magnesium treatments are relatively larger, while the inclusion species increase significantly when calcium and magnesium are added simultaneously. The composite inclusions of magnesium–aluminum spinel as the core and calcium aluminate wrapped around were all formed, despite the differences in the order and ratio of calcium and magnesium addition, but there were obvious differences in inclusions evolution paths. Combing the inclusions distribution and properties evolution, it can be concluded that the treatment method of magnesium before calcium is the best, that is, a small amount of magnesium is added first to make inclusions fine dispersion, thus reducing the impact on steel performances, and then calcium is added to modify the surface of inclusions into liquid phase, which can effectively reduce the nozzle clogging.</p>