Abstract: Lap welding of 0.1 mm-thick beryllium bronze with nickel interlayer was realized using micro-resistance spot welding. Tensile shear test, optical microscopy (OM) scan electric microscopy (SEM) and energy spectrum analysis (EDS) were used to study the influence of Ni interlayer on the dissimilar joint formation and the joints’ shear strength. The results show that, mechanism of beryllium bronze with nickel interlayer joint formation were solder bonding and fusion-solder bonding; The formation of the joint has gone through four processes: copper alloy wetting and spreading, element diffusion, nickel-copper interface reaction and metal solidification. Under the two kinds of joint formation mechanism, the fracture mechanism of solder bonding is fracturing along the interface, and it’s button fracture when fusion-solder welding. Their fractures present ductile fracture with a mixed characteristic of brittle fracture.