Abstract: Based on the results of actual engineering investigation,Common air environ-ment is divided into two class,indoor and outdoor or indoor moist environment.Using the theo-ry of corrosion eletrochemisty,a mathematical model of steel corrosion predicted is given.In consideration of the influence of air relative humidity and ability to absorb oxygen for water membrane,the formula to calculate corrosion extent is established before and after concrete breaking.Then,the author analyzes relationship of steel corrosion extent and every influence factor.