Magnesium in raw materials is one of the important factors affecting the mineral structure of high-alkalinity sintered ores. In order to reveal its influence mechanism and enrich the mineralization theory of high-alkalinity sinter， chemical pure reagents is used as the main raw materials，micro-sintering is carried out in the laboratory，and the occurrence state and migration law of magnesium in high-alkalinity sinter at different metallogenic stages are analyzed with the help of polarizing light microscope and electron probe. The results show that the change of w( MgO) has a significant impact on the content and morphology of various minerals in sinter during sintering mineralization. The increase of w( MgO) ( 1. 0%~ 3. 0% ) can promote the formation of magnetite and inhibit the formation of hematite，calcium ferrite and glass phase. At different metallogenic stages，the microstructures of sintered ores with different w( MgO) in the raw materials are loose，molten gel，melted，maculosus，skeletonic & crystalline，and interwoven-fused. In the heating stage ( room temperature ~ 1 400 ℃ ) ，magnesium begins to migrate into the mineral lattice of sinter at 1 280 ℃，and is mainly present in magnetite and calcium ferrite. With the increase of water quenching temperature to 1 400 ℃，the magnesium elements in calcium ferrite and glass phases gradually migrates to magnetite. During the cooling process ( 1 400 ~ 1 100 ℃ ) ，magnesium gradually migrates into magnetite and calcium ferrite，and mainly exists in magnetite. At different water quenching temperatures，the w( MgO) in magnetite and glass phases is directly proportional to the w( MgO) in the raw material，while the w( MgO) in hematite and calcium ferrite does not change significantly with the w( MgO) in the raw material.