Abstract: In-situ observation and OTS analysis were used to analyse the formation, morphology and elemental distribution law of inclusions in the welded heat-affected zone（HAZ） of AH36 steel under different heat inputs of the welding process, and to reveal the mechanism of the influence of the evolution of inclusions on the nucleation of acicular ferrite, as well as on the toughness of the welded heat-affected zone. The results show that: with the increase of heat input, the impact energy of the test steel increases and then decreases, and when the heat input is 550 kJ/cm, the value of the impact energy is higher and the toughness is better. The microstructure of the weld heat affected zone of the test steel is mainly composed of acicular ferrite（IAF） and massive ferrite（PF）. The austenite grain size of the test steel is 85-200 μm, which can promote the nucleation of acicular ferrite, and the contribution of intracrystalline acicular ferrite to the improvement of the toughness of the heat-affected zone is greater compared with that of grain boundary acicular ferrite. Based on the theory of manganese-poor zones and the inert interface mechanism, the inclusions in the heat-affected zone of the test steel induce the nucleation of acicular ferrite, which is the reason for the good toughness of the heat-affected zone. Test steel inclusions can be divided into two categories, one is the size of 1.5-2.5 μm inclusions in large quantities, the other is a size range of 2-4μm complex inclusions（Al, Mg, Ti, Ca, Si）-O or（Al, Mg,Ti）-O. The coexis tence of the two are prone to forming inclusions induced by inclusions in the crystal acicular ferrite nucleation, and polygonal ferrite uniform distribution, which is the reason that 550 kJ/cm test steel welding heat affected zone has good toughness.