Abstract: The reversed austenite growth mechanism and low temperature impact toughness of coarsen-grain heat-affected zone(CGHAZ) were studied through methods of welding thermal simulation and high-temperature laser scanning con-focal microscopy(LSCM) for high-Nb fire-resistant steel. The chemical composition system that was designed with low-Mn, high-Nb and super low-C increased the reverse austenite transformation temperature and shortened the austenite growth time on heating of welding thermal cycle. LSCM showed that the reversed austenite grew in the mode of grain boundary migration with the low growth rate at the first growth stage on heating of welding thermal cycle. Then it grew in the mode of grain merging without growth mode of swallowing up, and the austenite grains did not grow up on cooling of welding thermal cycle. Thus, austenite grains were not coarsened largely in the investigated steel. The grain size was around 24 ～ 41 μm in submerged arc welding CGHAZ under heat input of 15 kJ/cm, 50 kJ/cm, 75 kJ/cm. The fine grains increased the impact toughness at-40 ℃ and the impact absorption energy was greater than 240 J.