Abstract: Taking Ti-Mg deoxidized steel as research object, the thermal simulation and confocal scanning laser microscopy(CLSM) were used to study the influence of different cooling speeds on the phase transition of the structure and the in-situ observation of the formation process of intragranular inclusions induced acicular ferrite under continuous cooling conditions.The results show that as the cooling rate increases, the content of acicular ferrite first increases and then decreases. When the cooling rate is 5 ℃/s, the content of acicular ferrite(AF) is highest. The high temperature in-situ observation shows that AF starts nucleation at inclusions at the position of 614 ℃, and the nucleation growth ends at 553 ℃. The inner layer of the electron probe surface scanning inclusions is mainly MgO-Al₂O₃, and the outer layer is covered with Ti Ox-MnO. The formation of Mn-deplete zone(MDZ) is the main reason for inducing Ti-Mg composite inclusions to induce the formation of AF.