In this paper, the corrosion specimens are obtained by ways of constant temperature and humidity ,and the size of corrosion pits is detected by roughness tester. So surface characterization of steel corrosion is researched by fractal theory and it has been proved by structure function that it has the fractal character. It find that the fractal dimension is inherent and it will not change with the sample length. The result indicates that the fractal dimension is decreasing with the corrosion rate increasing. so it will help to solve the difficult of safety assessment for In-service steel structure.