Abstract: During the multi-layer and multi-pass welding, the heat affected zone, especially the original coarse grained zone beyond the fusion line, will be heat affected by subsequent welding beads. And the mechanical property will be promoted. This article adopts the numerical simulation technology to analyze the microstructure evolution of coarse grained zone during the single and tandem wires narrow gap submerged arc welding. Simulation results show that, if weld thickness is 4 mm, 50% of the original coarse grain zone becomes fine grained during the single wire welding and 55%~60% of the original coarse grained zone becomes fine grained during the tandem wires welding. Reducing the leading wire's heat input can obtain more smaller coarse grained zone.