Sulfate is an important secondary inorganic component of haze particles, its formation mechanism, however, is still not completely clear. Besides, current air quality models often underestimate the sulfate formation amount, and cannot reproduce the field observation results successfully. Hence, it is essential to give insight into the atmospheric sulfate formation processes, which contributes to the understanding of haze formation mechanism and precise control of haze pollution. In this study, the non-catalytic oxidation process of SO2 with NaCl droplet was observed by optical tweezer-stimulated Raman spectroscopy. The nanoscale change of droplet radius during the reaction process can be accurately measured, so that the real-time reaction process can be in situ observed. According to the measured reaction rate, the pseudo first-order rate constant k was calculated, and the effects of relative humidity and SO2 concentration on the reaction were also investigated. Results showed that the change of droplet ionic strength caused by varied relative humidity significantly affected the k value. When the humidity increased from ~60% to ~90%, the k value decreased by an order of magnitude. As to the effect of SO2 concentration, increased SO2 concentration decreased the pH of the droplet, leading to a slight decrease in the k value. Based on the further discussion of the reaction mechanism, we found both bulk phase reaction and interface reaction may contribute to the non-catalytic oxidation of SO2 with NaCl droplet.