Abstract: The coil skeleton parts(electrical pure iron+stainless steel) welded by diffusion welding have the advantages of flat weld interface, high dimensional accuracy and excellent welding quality, it is widely used in hydraulic steering gear products. During the development of a new coil skeleton product, it was found that the height of the blank parts after diffusion welding was too much out of tolerance, the electrical pure iron material in the parts had a serious upsetting deformation and could not be used. According to the research, the difference in the purity of electrical pure iron is the root cause of the problem. Under the same diffusion welding parameters, the higher the purity of electrical pure iron, the worse the rigidity of the material in the diffusion welding process, and the more severe the deformation. By adding BNi82CrSiBFe intermediate layer and optimizing the process parameters, the tensile strength of diffusion weld has been greatly improved on the basis of ensuring the deformation rate of the blank parts after welding, and it reaches more than 320 MPa, which meets the use requirements of the product.