The D614G mutation in the spike protein of SARS-CoV-2 alters the fitness of the virus, leading to the dominant form observed in the COVID-19 pandemic. However, the molecular basis of the mechanism by which this mutation enhances fitness is not clear. Here we demonstrated by cryo-electron microscopy that the D614G mutation resulted in increased propensity of multiple receptor binding domains (RBDs) in an upward conformation poised for host receptor binding. Multiple substates within the one RBD-up or two RBD-up conformational space were determined. According to negative staining electron microscopy, differential scanning calorimetry and differential scanning fluorimetry, the most significant impact of the mutation lies in its ability to eliminate the unusual cold-induced unfolding characteristics, and to significantly increase the thermal stability under physiological pH. The D614G spike variant also exhibited exceptional long-term stability when stored at 37 ºC for up to two months. Our findings shed light on how the D614G mutation enhances the infectivity of SARS-CoV-2 through a stabilizing mutation, and suggest an approach for better design of spike protein-based conjugates for vaccine development.