The method of digital image processing is used to study the damage of cavitation on the surface of the aerofoil subarea. The cavitation corrosion's images of the aerofoil are gained with SEM. First, fuzzy enhancement is used. Second, threshold analysis is applied to pick-up the shallow pit. Third, morphology method is used to process the edge and work out the cavitation values. The idiographic numerical and quantitative values such as cavitation area, quantity are obtained.