Abstract: In view of the high heat input welding steel produced by a domestic steel plant, the liquid steel samples of refining (ladle furnace refining, liquid steel vacuum circulation degassing) and continuous casting process nodes were taken. The evolution of quantity, size, composition and type of Mg-Al-Ti composite inclusions in high heat input welding steel were investigated with ASPEX and JSM-6510LV scanning electron mcroscopy. The results show that the content of Al₂O₃ in inclusions decreases, while the content of MgO increases, and the type of inclusions is changed from silicon manganese oxide to Mg-Al-Ti composite inclusions in the process of ladle furnace refining. After adding Ca in ladle furnace refining, the content of Al₂O₃ in inclusions increases and the content of MgO decreases. There is no obvious change in inclusion types from vacuum degassing to continuous casting, and the contents of Al₂O₃ and MgO in inclusions has little obvious change, while the contents of CaO and CaS increases. The inclusions with particle size of 0.5-5 μm account for more than 95% in the continuous casting, and the inclusions mainly exist in the form of Mg-Al-Ti composite inclusions.