It is well known that quality of rainwater (e.g., acidity) is dependent on the chemical substances which originate from such scavenging processes in the atmosphere as rainout (in-cloud) by cloud droplets and washout (below-cloud) by rain drops. Rainout plays an important role in the acidification of rainwater, as the first stage of scavenging process. However, characteristics of rainout process have not been made clear yet both theoretically and experimentally, because the accurate field data on cloud droplet quality are quite scarce. In order to estimate the nonsteady acidification of cloud droplets and the contribution of rainout to acid rain formation, the cloud droplet acidification caused by absorption of SO2 (g), H2O2 (g) and HNO3 (g) is investigated numerically, using a mathematical model based on the physicochemical considerations. The cloud droplet acidification process are implemented by lumped differential equations. From the numerical results it is concluded that:(1) the dominant species for the acidification in the cloud droplets is changeable as HSO3- in very early period, and then NO3- during next three hours, and thereafter SO42-;(2) droplet-phase oxidation of HSO3 plays an important role for droplet acidity, even in the case that strong acidifier of HNO3 (g) coexists; and (3) even after one-day rainout most of SO2 (g) remains in the air-phase, but most of HNO3 (g) is consumed within one-minute rainout.