Abstract: According to in-situ weightlessness tests at Chengdao region, the atmospheric corrosivity was classified by ISO 9223 standard. The corrosion rates of carbon steel in marine atmosphere were more than 100 times the rates in land's atmosphere. The indoor corrosion under marine environment was about ten times lower than outdoor corrosion, because of difference of salt deposit. The salt deposits on corrosion coupons were measured by methods of iron-selection electrodes. The salt deposits on outdoor coupons were 40 times higher in average than on indoor coupons. The influences of salt deposits on corrosion of carbon steel were investigated in analogue environments by means of corrosion detective cells. The regression analysis showed that the atmospheric corrosivity had liner relationship with salt deposits in logarithm scales. Small amount of salt deposites make the corrosion of carbon steels increased in orders of magnitude, with critical relative humidity to move down from 60% to 40%.