Abstract: With the comprehensive trend of microelectronic packaging towards lead-free, it is becoming increasingly miniaturized, portable, and multifunctional. The growth of tin whiskers poses a serious threat to the reliability and safety of electronic products in service. In this study, the molecular dynamics models with and without grain boundaries were built to explore the growth process of tin whisker of copper-tin double coating. The growth process of tin whisker was simulated by thermal cycling and grain boundaries under multi-physical field (thermo-mechanical) coupling. The effects of external force, different thermal cycles (0, 5, 10) and grain boundaries on the growth of tin whiskers and the variation of internal stresses were investigated. The results show that in a certain range, the increase of thermal cycles will promote the growth of tin whisker, and the growth rate of tin whisker is faster in the model with grain boundary. The internal stress at the bottom of tin whisker reaches the maximum, forming a stress gradient to promote the growth of tin whisker. The external load will increase the internal stress of copper-tin double coating, and the accumulation time of internal stress will be shortened by about 1.5 ns, accelerating the growth rate of tin whiskers.