The invention belongs to the field of detection of anticorrosion coatings and provides a three-electrode potential difference type detection method for an inner coating of a tubing and casing in a downhole. The method includes: descending a positive electrode and auxiliary matched instrument strings into the tubing and casing, providing constant voltage for the positive electrode, and acquiring potential difference between a lower test electrode and an upper test electrode in a process of lifting the instrument strings out of the downhole, wherein the potential difference reflects quality of the inner coating of the tubing and casing. The three-electrode potential difference type detection method for the inner coating of the tubing and casing in the downhole has the advantages that under the condition of existence of conductive media such as oil, water and the like in oil-water wells, acquired data can be processed and interpreted to realize accuracy in qualitative judgment and quantitative judgment of the inner coating of the tubing and casing in the downhole, and high precision, quickness in logging, convenience and safety in operation and low detection cost are realized.