Developing low temperature annealing processing for preparing high reliable cyanide-free electroplated Ag–Sn films as a promising reflective coatings has a great potential industrial application prospect. The morphology evolution of Ag4Sn is in situ studied by synchrotron radiation real-time imaging technology. The phase evolution behavior of Ag4Sn in Ag/Sn films are revealed using X-ray diffraction quantitative analysis technology. The feasibility to prepare high reliable Ag/Sn films at 358 K has been confirmed by the systematic studies on the evolution of different phase contents and reflective performance of cyanide-free electroplated Ag/Sn films as a function of low temperature annealing time. Besides, we collect and calculate the fundamental thermodynamic property data of Ag–Sn system. The effect of nucleation positions on the formation and growth of intermetallic compounds types are discussed based on the thermodynamics analysis. This paper opens a new window to obtain high quality Ag/Sn reflective films at low annealing temperature and provides a guiding principle for the fabrication of Ag/Sn reflective materials for LED chips as well as coating materials for organic soft optoelectronic devices.