Abstract: Objective As a core equipment in infrastructure construction, the mast welds of rotary drilling rigs are prone to stress concentration and defects under extreme working conditions, which directly affects construction safety and equipment service life. To realize the determination of the welding quality of rotary drilling rig mast welds, this paper uses X-ray tests to conduct an in-depth study on the defect identification of rotary drilling rig mast welds. Methods Firstly, by simplifying the mast structure model and analyzing its stress distribution and deformation under different working conditions, the large disc weld is determined as the most dangerous location for X-ray inspection. Various image filtering and enhancement methods are compared, and a preprocessing algorithm based on SUNet filtering and linear enhancement is determined, which significantly improves image quality. Further, by studying the threshold segmentation and edge detection technologies of weld images, Otsu’s threshold method and Canny operator are combined to effectively extract key image features. On this basis, common weld defects are analyzed, and a dataset containing 24 407 images of 224×224 pixels is constructed, covering four types of samples, cracks, pores, incomplete penetration, and non-defective ones, which are divided into a training set, a validation set, and a test set in proportion. A residual recognition network is designed, and combined with sliding window technology, a defect identification algorithm suitable for various image sizes is proposed. Results This algorithm can achieve a precision of 0.977 89, an accuracy of 0.981 88, a recall of 0.98186, and a F₁ parameter of 0.979 33 in defect identification, reaching an accuracy rate of 89% in weld X-ray inspection images of different sizes, and has been successfully applied in rotary drilling rig X-ray inspection images. Conclusion The algorithm shows high adaptability and precision in the defect detection of rotary drilling rig mast welds, achieving good detection results. It provides an efficient and high-precision non-destructive testing scheme for rotary drilling rig weld defects, effectively improving the safety and reliability of equipment operation and maintenance, and is of great significance for the quality assurance of infrastructure projects.