The surface treatment of Q345 steel plate was carried out by plasma spray welding. The optimum process parameters of Q345 surface treatment were optimized by orthogonal experiment. The optimized spray welding process was as follows: current 60 A, welding gun walking speed 20 mm / min, powder feeding rate 35%, ion gas flow rate 1.2-1.4 L / min, shielding gas flow rate 10-12 L / min. The results show that the microstructure of Q345 substrate is normal, which is pearlite + ferrite. The microstructure of heat affected zone is still pearlite + ferrite. The microstructure of spray welding layer is iron matrix and eutectic (Cr, Fe)₇C₃ phase. ZrC in the spray welding layer can be used as the heterogeneous core of iron matrix, which can refine the grain size of iron matrix and improve the mechanical properties greatly. The impact toughness of the composite plate prepared by the optimal process is 192.8 J/cm². The fracture mode of the high strength layer is quasi cleavage fracture, and the fracture mode of the matrix layer is cleavage fracture.