Abstract: The double-wall tube can control the crack propagation path and prevent the crack from spreading along the tube diameter, which significantly improves the safety of the tube. Therefore, it is the ideal choice for the cooling tube in the breeding blanket zone of fusion reactor. The process optimization experiments of steel/copper/steel and steel/nickel/steel flat composite structures were carried out to solve the problem of double-wall tube. The results show that grain boundary diffusion occurs at steel/copper/steel composite interface when the hot isostatic pressing temperature is higher than 1 100 ℃. The room temperature tensile strength of the welded joint reaches 601 MPa and the interface nanoindentation hardness is 1.37 GPa. Wider solid solution diffusion of elements occurs at the steel/nickel/steel composite interface, the tensile strength at room temperature can reach 630 MPa, and the nanoindentation hardness at the interface is 2.05 GPa. Prototypes of multi-bending double-wall cooling pipes with good connection quality were achieved by using optimized hot isostatic pressing parameters.