Abstract: To address issues such as nozzle clogging and steel performance degradation caused by individual calcium or magnesium treatment, this study proposes a combined calcium-magnesium treatment method. First, the FactSage thermodynamic software was used to analyze the effects of different calcium-magnesium ratio combinations on the evolution of inclusions in molten steel at 1 873 K. Subsequently, high-temperature smelting experiments were conducted using a vacuum induction furnace, during which Si-Ca alloy and Ni-Mg alloy were simultaneously added (the total amount was fixed, and the mass fraction ratios were 80%Ca+20%Mg, 50%Ca+50%Mg, and 20%Ca+80%Mg, respectively). Process samples were taken at 1, 3, 5, and 10 minutes after the addition, and the morphology and composition of inclusions were analyzed by scanning electron microscopy to investigate the effect of calcium-magnesium composite treatment on the modification behavior and microstructure of inclusions. The results show that the inclusion liquid area is the largest in the 80%Ca+20%Mg composite treatment. With an increasing proportion of magnesium alloy, the inclusion liquid area gradually decreases. The evolution path of inclusion composition with time is basically the same for the three proportions, and finally forms a layered structure with magnesium-aluminum spinel in the center and calcium aluminate on the outside. Increasing the amount of magnesium alloy in the composite treatment helps to increase the number density of inclusions and reduce their size. For example, in the 20%Ca+80%Mg experiment, the number density of inclusions reaches 272 N/mm² at 1 min after addition, and the average inclusion size is the smallest at 10 min, which is 1.2 μm. The magnesium-aluminum spinel that forms after the addition of magnesium alloy serves as nucleation particles, which facilitates a fine and dispersed distribution of inclusions, while calcium treatment modifies some inclusions into the liquid phase zone, thereby helping to reduce nozzle clogging. This study can provide a theoretical basis and technical reference for improving the comprehensive performance of ship plate steel.