Abstract: To meet the welding technology requirements for the batch manufacturing of key structural components used in high-pressure large-diameter oil and gas pipeline leak excavation equipment, this study focuses on the commonly used dissimilar steel joints in such equipment. The welding process and joint performance of 35SiMn steel and 800 MPa grade high-strength steel were investigated. These load-bearing components are subjected to long-term cyclic loading and instantaneous impact, imposing high demands on the strength, toughness, and structural stability of the welded joints. Gas metal arc welding (MAG) was employed, and two types of filler wires, ER70S-G and ER110S-G, were selected to conduct butt welding tests on 12 mm thick test plates. The mechanical properties of the joints, including tensile strength, bending performance, and hardness distribution, were systematically evaluated. Combined with microstructural observations, the relationships among welding process, microstructure, and joint performance were analyzed. The results show that the joints welded with ER110S-G wire exhibit an ultimate tensile strength of no less than 849 MPa, matching the strength of the 800 MPa grade base metal, and demonstrate excellent comprehensive mechanical properties. Bending tests indicate sound bonding integrity in all joints. Hardness measurements reveal a mild hardness gradient in the heat-affected zone (HAZ) of ER110S-G welded joints, effectively reducing the risk of structural stress concentration. Microstructural analysis shows that in the coarse-grained HAZ on the 800 MPa grade steel side of ER110S-G welded joints, a low-carbon martensite phase structure dominated by sorbite and bainite is formed, while on the 35SiMn side, the microstructure is mainly sorbite and bainite; the fine-grained HAZ of both sides consists of sorbite, pearlite, and bainite. Such uniform microstructure distribution is the underlying reason for the gentle hardness profile and stable performance of these joints. In contrast, ER70S-G welded joints exhibit lath martensite and bainite in the coarse-grained HAZ on the 35SiMn side, which tends to cause locally high hardness and is detrimental to long-term service stability. The findings of this study can provide a reference for welding process selection, standardized production, and material design for key load-bearing structural components of such pipeline excavation equipment, thereby contributing to enhanced overall service reliability and service life.