The microstructure of low-Ag-content Sn-2.5Ag-0.7Cu(0.1RE)/Cu solder joint interface and growth behavior of Cu6Sn5 intermetallic compound (IMC) were investigated by XRD, SEM and EDAX. The results show that the Cu6Sn5 thickness of the solder joint interface is decided by its dissolution and growing during soldering. With the aging time increasing, the Cu6Sn5 morphology of the solder joint interface changed from scallop-like to lamellar; the growth dynamics follows the parabola law and its growth behavior is controlled by diffusion mechanism. Addition of 0.1% RE (mass fraction) in the Sn-2.5Ag-0.7Cu solder alloy can effectively reduce the growing rate of the solder joint Cu6Sn5 IMC during the soldering and aging period, and change the fracture mechanism of solder joint; therefore the reliability of the solder joint can be greatly improved.