The corrosivity inside box girders of steel bridges was investigated. The humidity controlling and rust prevention using a special desiccant were studied. The desiccant used in this study is capable of absorbing and releasing the moisture at high and low humidity respectively. The effects of humidity controlling and rust prevention of the desiccant were confirmed by the field test inside the box girders. On the basis of above investigations, a rust preventing system was proposed, in which the desiccant was used instead of painting inside the box girders. The life-cycle cost for the rust prevention of steel box girders can be remarkably reduced by this rust preventing system.