Corrosion of prestressing strands is a main durability issue for prestressing structures. In general, the corrosion zones of strands are hidden in structures which lead to be hardly inspected by traditional methods. An instrument for detecting the non visible corrosion of prestressing strands using magnetostrictive guided waves is provided. The instrument is described in detail. The workflow to detect the strands is provided. Field trials and laboratory experiments show that the information of position, length and amount of the corrosion can be obtained from the data. The instrument provides support for ensuring the safety of prestressing structures.