Abstract: The multi-layer and multi-pass welding of G115 steel plates was carried out by manual arc welding under two heat input conditions (low and high). The low heat input condition was set as 21.07 kJ·cm⁻¹ for root welding and 11.05 kJ·cm⁻¹ for filling and capping welding, while the high heat input condition was set as 25.38 kJ·cm⁻¹ for root welding and 15.05 kJ·cm⁻¹ for filling and capping welding. The influence of heat input on the microstructure and mechanical properties of the joint was studied. The results show that the joints under two heat input conditions consisted of base metal, heat-affected zone and weld, and the heat-affected zone could be further divided into overheated zone, phase transformation recrystallization zone and incomplete recrystallization zone. The microstructures in different zones were similar. Compared with those under the high heat input, the average width of the heat-affected zone under low heat input joint was narrower, and the grain size of the weld and the heat-affected zone was smaller. After tension at room temperature, the joint fractured at the weld under different heat inputs. Compared with those under the high heat input, the tensile strength of the joint under the low heat input changed little, the percentage elongation after fracture increased significantly by approximately 33%, the impact absorption energy of the heat-affected zone and the weld decreased by about 18.0% and 7.2%, respectively, and the hardness was slightly lower. The bending specimens of the joints under the low heat input did not fracture, while those under the high heat input showed fracture or cracking. The joints obtained under the low heat input condition had relatively excellent comprehensive performance.