The segregation of Bi phase during current stressing deteriorates the mechanical property of Sn-58Bi solder joint. To improve the reliability of Sn-58Bi solder joint, a thorough understanding of the diffusion behavior of Bi elements is necessary. In this study, a Cu/SnBi/Sn-xCu/SnBi/Cu multi-layer solder joint structure was adopted to reveal the diffusion path of Bi element and the effect of Cu on the diffusion behavior of Bi during electromigration. The result showed that the Bi element diffused along the grain boundary. The presence of Cu had two competing mechanisms on the diffusion of Bi: it slowed the process by forming Cu6Sn5 intermetallic compounds that blocked and prolonged the diffusion path, yet also accelerated it by refining the Sn and creating more diffusion channels. A data processing method was used to estimate the diffusion rate of Bi and determine the dominant mechanism. The results indicated that the diffusion rate was accelerated by Cu doping, confirming that the dominant mechanism was the refining effect, which promoted the diffusion of Bi element.