Abstract: Influence of different boron contents on the formation of acicular ferrite in weld metal which was welded by flux cored wire for high strength steel was analyzed. Impact energies of weld metals at different test temperatures were acquired by metallographic observation, impact test and dilatometric test. Through thermodynamics and dynamics view, the influencing mechanism of the boron element on the formation of acicular ferrite was analyzed by transmission electron microscopy analysis and interrupted quench experiments methods. The results showed that the free boron element in weld metal grain boundaries suppressed the grain boundary ferrite formation and was beneficial to acicular ferrite nucleation. The free boron contents in the grain boundaries reduced with increasing nitrogen contents in weld metal. The transformation temperature of austenite to ferrite increased. The acicular ferrite content reduced. The acicular ferrite nucleated and grew on multiphase inclusions which took the titanium oxide and manganese oxide as the core, the copper sulfide and manganese sulfide as the hull, and the boron nitride as the transition layer. Increasing the acicular ferrite contents in weld meal was beneficial to improving the impact energy of weld metal. The maximum impact energy at -60 °C was 70 J.