Underwater concrete structures in hydraulic dams are prone to defects. Traditional manual inspection methods are inefficient and carry high risk factors. Existing underwater 3D reconstruction techniques struggle to achieve high precision due to poor image quality and complex aquatic environments. Therefore, for underwater structural defect detection, this paper proposes a 3D reconstruction solution integrating a multi-scale fusion image enhancement algorithm with an underwater refraction model. This approach combines adaptive color correction and an improved Retinex algorithm through non-subsampled shearlet transform (NSST） algorithm-based multi-scale fusion to optimize image quality, then utilizes the underwater refraction model to complete 3D reconstruction. Experiments demonstrate that this method outperforms traditional underwater image enhancement models in terms of information entropy (IE), patch-based contrast quality index (PCQI) and underwater color image quality evaluation (UCIQE) metrics.The processed images achieve an optimal IE value (7.803 6), with actual reconstruction results showing optimal detail textures and edge contours. This approach effectively enhances the reconstruction accuracy of underwater structural defects, providing reliable support for underwater inspection.