The effects of ytterbium (Yb) on the thermal behavior, wettability, and microstructures of Ag30-Cu-Zn-Sn (BAg30) filler metals were investigated. The shear strength and fracture morphology of brazed joints were analyzed to optimize the filler composition. Results reveal that the addition of 1wt% Yb in BAg30 filler metal can significantly decrease the temperature difference between solidus temperature and liquidus temperature, improve the wettability of filler on the steel substrate, and refine the microstructures. However, the excessive addition of Yb (more than 1wt%) can degrade the properties and coarsen the microstructure. According to the shear strength tests, the BAg30-1wt% Yb brazed joint shows obvious superiority in shear strength and fracture morphology: its shear strength is higher than that of the original BAg30 brazed joint, and the fracture mechanism is ductile fracture, presenting the homogeneous and fine dimples.