Abstract: The 35 kg ingot was refined with vacuum induction furnace in laboratory. The high heat input welding thermal simulation test was conducted in 100 kJ/cm heat input at 1400°C top temperature by using thermal simulator MMS-300. The microstructure was observed, and the effect of number, size, distribution and composition of oxide inclusions on the formation of acicular ferrite in coarse grain heat affected zone (CGHAZ) was studied with optical microscope (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and electron probe micro-analyzer (EPMA). The results showed that the fraction of acicular ferrite in CGHAZ increases with the number and area fraction of oxide inclusions. Acicular ferrite forms on Ti, Zr complex inclusions, the inclusions are mostly Ti, Zr oxide as the core, and MnS precipitates on the surface of oxide inclusions, with size in the range of 0.5~3 μm.