The interfacial reactions and growth kinetics of Sn-20Bi-0.7Cu-xAg (x=0.1, 0.4, 0.7, 1.0, 1.5, wt%) were investigated during solid-state aging. The effects of chemical composition on the structure and the growth of interface under high temperature and high humidity conditions were studied experimentally and numerically. In order to determine the long-term reliability of the solder joints, thermal accelerated aging tests were performed for 0, 10, 30, 50, 100, 200 and 500 h under 85 °C and 85% relative humidity conditions. The surface morphology, thickness, and distribution of interface compounds were observed by scanning electron microscope. The nucleation surface and growth direction of β-Sn were clarified. The phases were determined at the interface and the alloy matrix. Results show that with increasing the Ag content, the growth of the Cu₆Sn₅ layer is suppressed. The growth kinetics of intermetallic compound (Cu₆Sn₅+Cu₃Sn) remains a diffusion-controlled process during the isothermal aging. The scallop-type Cu₆Sn₅ phase disappears at later aging stages, suggesting that the growth mechanism changes to the steady growth in the direction perpendicular to the interface. Besides, the results reveal that Ag₃Sn effectively slows down the growth kinetics of Cu₆Sn₅.