Cracks under coating of flow passage parts of hydropower station is a very serious fault, which can directly lead to fatigue fracture. This will directly affect the safety of the entire hydropower unit. The array eddy current testing method has been studied, and crack signals are found with a lot of noise. In order to improve the signal quality, adaptive second generation wavelet transform is applied to the array eddy current signal analysis to reduce the signal noise. A test block with some cracks under the coating was tested, and the tiny crack defect with size of 0.13mm × 0.5mm × 5mm was identified. The experimental results show that the array eddy current detection method based on adaptive second generation wavelet transform can effectively identify the cracks under the coating of flow passage components of hydropower station, with high detection accuracy and strong adaptability, and has good engineering application value.