The microstructure of Sn35Bi-xCo(x=0%, 0.3%, 0.7%, 1.0%, 1.2%, 1.5%, mass fraction) composite solder/Cu joints was observed by scanning electron microscopy (SEM).Combined energy spectrum (EDS) and XRD analysis, the difference of joint structure was studied. The mechanical properties of joints were tested by universal testing machine, and the influence mechanism of Co particle content on the structure and properties of SnBi/Cu joints was studied. The results show that, with the increase of Co particle content, the wettability of Sn35Bi-Co composite solder increases first and then decreases. When Co particle content is 0.7%, the wettability is the best.When appropriate amount of Co particles is added to the Sn35Bi/Cu joint in the solidification stage, the weld microstructure can be effectively refined, the IMC layer of the interface is more flat, Co atoms in the weld replace Cu atoms in the interface Cu₆Sn₅ layer, and the (Cu, Co)₆Sn₅ solid solution can be formed, which can strengthen the IMC layer of the interface.The shear strength of Sn35Bi-Co/Cu joints increases first and then decreases with the increase of Co particle content. When Co particle content is 0.7%, the maximum value of 54.09 MPa is obtained.