To overcome AgCuZnSn filler metal with high tin content is difficult to form for its high brittleness, a composite flux-cored silver filler metal with CuSn alloy powder core was designed based on the concept of flux-cored wire. The effects of 30wt%~70wt%Cu60Sn40 alloy powder on the wettability, microstructure, interfacial microhardness and tensile strength of silver filler metal, copper/Q235 steel brazing joint were studied. The results show that AgCuZnSn filler metal with high tin content is in-situ synthesis by CuSn alloy powder in core and BAg30CuZnSn of outer layer during brazing process, and uniform composition with good joint structure was obtained. With the increase of the content of alloy powder in the core, the wetting area of composite flux-cored silver solder on copper plate and Q235 steel plate is promoted constantly, the microhardness of copper/Q235 steel brazing joint is rised continuously, and the tensile strength is increased first and then decreased. When the content of CuSn alloy powder in the powder core is 30wt%, the maximum tensile strength (198.91MPa) of Cu/Q235 steel brazing joint is obtained, which is enhanced by 23.4% compared with the core silver brazing filler metal without alloy powder added.