<p>Unveiling the morphological transition mechanism of eutectic structures in the Ag-based filler metals is crucial to control the microstructures in the brazing seam and improve the mechanical properties of brazed joints. In this study, the effect of alloying elements addition (Co, Mn, Ni, and Ti) on the morphological evolution in the Ag-based filler metals was investigated, and the morphological transition mechanism of eutectic structures after addition of different contents at various cooling rates was elucidated. The variation in addition content of alloying elements and solidification velocity strongly affected the formation and morphologies of Ag dendrites and Ag–Cu binary eutectic structures. For the low content of alloying element and slow solidification velocity, the FES and LES always coexisted in the samples, indicating a competitive growth. With increasing content and/or solidification velocity, the LES → FES, FES → LES, and LES → AES transitions were determined. The AES formation with four different patterns including AES at the intersections of FES clusters, AES at the intersections of LES clusters, AES resulting from Cu-rich phase precipitating between Ag dendrites, and between secondary dendritic arms. The AES formation was mainly attributed to the recalescence during solidification promoting the partial remelting of FES or LES, and the precipitation of Cu-rich phase splitting Ag dendrite skeleton. The result provides a deep insight into the microstructural evolution in the Ag-based filler metals and the brazing seam and offers guidelines for controlling the microstructure and properties of brazed joints by modifying the brazing filler metal composition and brazing process parameters.</p>