Abstract: High purity copper electrode was used to braze 10 steel with brazing seam gap in the range of 0.05‒1.00 mm. The effects of braze seam gap on the formability, interfacial microstructure and mechanical properties of brazed joints were studied. The results show that when the brazing seam gap was less than 0.10 mm, the solder could not be fully absorbed into the gap, resulting in poor local infiltration. When the brazing seam gap was 0.10‒0.30 mm, the interface of brazed joints was well infiltrated and the brazing seam was full. When the brazing seam gap was too large (greater than 0.50 mm), the brazing metal collapsed after melting, resulting in the joint filling was not enough to form a continuous brazed joint. When the brazing seam gap was 0.15‒0.30 mm, the tensile strength of the joint was 324‒345 MPa, when the brazing seam gap was too small or too large, the tensile strength was low. During the brazing process, Fe and Cu elements infiltrated into the brazing interface, and in the subsequent cooling process, the dispersed Fe particles were precipitated at the brazing interface near the brazing alloy side, and the grain boundary near the base metal side was thickened. No tearing was observed in the tensile specimens. Considering the brazing formability and joint strength, the brazing seam gap of 10 steel was suitable to be controlled in the range of 0.15‒0.3 mm.