Abstract: Three kinds of exothermic welding powders such as commercial powders, imported powders and self-developed powders were used to join the stainless steel clad steel materials which have broad application prospects in grounding grid. The microstructure of the joint and its formation mechanism were studied. The results showed that the stainless steel clad steel joints welded by the three kinds of exothermic welding powders show the similar microstructure characteristics. The obvious diffusion zone was found at Cu/steel interface. The typical interfacial structures in welded joint are Cu(s,s)/Cu(s,s) + Fe(s,s)/Fe(s,s) and the granular Fe-rich phase discretely distributes in the Cu(s,s) matrix. The formation of joint microstructure attributes to the dissolving and diffusing of steel into the molten copper after the exothermic chemical reaction. The results also revealed that the addition of active agents into exothermic powders plays an important role in purifying the joints and improves the followability of molted copper, which leads to the sound joint without pores and defects. On other hand, the addition of fluorite and SiGaBa powders can obviously enhance the slag removal function during chemical reaction process.