In this paper, iron-nickel alloy powder was prepared by adding 5 % nickel alloy powder into iron alloy powder. The iron / nickel mixed base spray welding layer was prepared on the surface of low alloy steel by plasma spray welding technology. The phase and microstructure of the Fe / Ni mixed-base spray welding layer were studied by orthogonal experiment using metallographic microscope, phase microscope and scanning electron microscope. To determine whether the bonding between the substrate and the spray welding layer is good and to observe and analyze the metallographic structure of the substrate, fusion line and spray welding layer. The software was used to analyze the metallographic phase diagram of the sample. Through the analysis of the hardness, forming coefficient, weld macro appearance, crack and other data of the sample, the Q345 composite plate with high hardness was optimized.