The effects of Mg content, inclusion size, and austenite grain size on the intragranular acicular ferrite (IAF) nucleation in heat-affected zone of steel plate after high-heat-input welding of 400 kJ/cm were investigated by welding simulation and observation using a scanning electron microscope equipped with an energy dispersive spectrometer and an optical microscope. The IAFs are observed in steel with Mg addition, and the volume fraction of IAF is as high as 55.4% in the steel containing 0.0027 mass% Mg. The MgO–Al₂O₃–Ti₂O₃–MnS inclusions with size around 2 μm are effective nucleation sites for IAF, whereas Al₂O₃–MnS inclusions are impotent to nucleate the acicular ferrite. The prior-austenite grain (PAG) size distribution in low Mg steel is similar to that in steel without Mg addition. The austenite grain with size about 200 μm is favorable for the IAF formation. In the steel with high Mg content of 0.0099%, the growth of PAG is greatly inhibited, and PAG sizes are smaller than 100 μm. Therefore, the nucleation of IAF can hardly be observed.