Abstract: In order to solve the problems of poor forming and coarse grain size caused by excessive arc heat input in the wire-arc additive manufacturing process, this paper conducted a study on forming and tissue optimization of bypass-current wire-heating plasma arc welding (PAW) additive manufacturing with H08Mn2Si carbon steel as the additive material. The effect of main/bypass current ratio on the fusion forming and heat input was investigated in a single-layer single-pass deposition test; then a multi-layer single-pass deposition test was conducted to analyze the forming, microstructure and hardness of carbon steel at different interlayer temperatures; finally, the tensile properties of the well-formed additive samples were tested. The results show that when the main/bypass current ratio is small, a uniform and smooth surface can be obtained, and the dilution rate of the base material can be reduced to 10%; when the interlayer temperature is controlled at a lower temperature of 100 °C, the surface quality of the formed additive is good, and the microstructure in the middle stable area of the specimen has a small grain size and an increased percentage of pearlite, and the highest average hardness can reach 294 HV. The tensile test shows that the strength and plastic properties are uniform in all directions, and the fracture form is ductile fracture.