Abstract: In order to study the influence of the element of boron and carbon on the microstructure and cracking sensitivity of the high boron content iron-based overlaying weld, the hybrid powder/wire overlaying weld technology was used to prepare the coating by adding different content of B4C. The structure and the mechanical performance of the coating were studied by the microstructure observation, micro zone element analysis, micro hardness and macro hardness. Results showed that the high boron content iron-based alloy consisted of α-Fe, Fe₂B and Fe₃(C,B) phase. With the increase of the content of B4C, the primary land-shape α-Fe began to disappear and the diamond-shape primary Fe₂B and granular Fe₃(C,B) began to precipitate. The volume fraction of the fish bone shaped and lath shaped eutectic microstructure (α-Fe+Fe₂B) was gradually reduced and finally disappeared. The main microstructure was characterized with flower-shape α-Fe+Fe₂B+Fe₃(C,B) peritectic structure in the overlaying weld. The content of boron and carbon and their ratio are the the key factor affecting the microstructure and the cracking sensitivity. The overlaying welding consisted of about 30% alloy powder (including 35% boron iron and 5% B₄C) and about 70% H08Mn2Si welding wire could effectively restrain the initiation of cracking. The hardness could reach a stable value of 66HRC.