Abstract: GLEEBLE 1500 thermal simulated test machine was used to simulate welding thermal cycle, optical microscope and scanning electron microscope were used to observe microstructure of thermal simulated samples, and precipitation law of carbide of X120Mn12 under welding thermal cycle was researched. The result showed that when t_8/5 was 6 s, there was discontinuous bar carbides precipitating in grain boundary. When t_8/5 was 15～30 s, precipitated carbides tended to grow up. When t_8/5 reached to 50～70 s, carbides at triangular grain boundary increased distinctly with large size and continuous distribution. Therefore, it was suggested that small welding heat input should be applied during arc welding, and t_8/5 should be kept within 30 s to avoid continuous carbide precipitating along triangular grain boundary in welding heat affected zone that reduced toughness of welding heat affected zone.