The invention discloses a microalloying method capable of improving performance of steel for large heat input welding. The specific flow process of the production method comprises a converter steelmaking process, an LF refining process and a continuous casting process; according to the kind and the adding sequence of a deoxidizing agent in the LF refining process, Mo, Mn, Si, Nb, Al, Ca, Mg and Ti are included; the Al is adopted for final deoxidation, Mo-Nb-Mg-Ti alloying is carried out, the dispersive action of the MgO on Al2O3 and Ti2O3 serves as the foundation, dispersed, rich and small refractory oxides are formed to serve as cores, and generation of acicular ferrite in austenitic crystalline grains is promoted; on the other hand, a certain amount of Mo and Nb improve toughness of the steel while the grains are refined, and forming of pro-eutectoid ferrite of the grain boundary is restrained; in addition, part of small inclusion particles and carbon nitrogen carry out pinning on the austenite grain boundary, through several-aspect joint action, heat affected zone grains are fully refined, and the toughness of the steel is obviously improved; and the method is simple in control and low in production cost, and the steel for large heat input welding can be subject to large-scale production in an industrialization manner.