When choosing the most suitable infrared thermal imaging detection scheme for online inspection during laser cladding processing, this paper designs the RespathU-net semantic segmentation defect detection network for cladding coating defects in infrared images. The network is based on the U-net network framework, and is optimized and improved by redesigning the coding network structure, expanding the network perceptual field, and connecting the paths of residuals, which improves the segmentation effect on the defective areas of the melt coating by improving the problems that the original network cannot realize the end-to-end output and the poor segmentation effect on the complex objects. Through the Kolektor SDD dataset and the infrared dataset constructed in this paper, the generalization performance test and defect detection experiment of the RespathU-net network were carried out, and the designed network was compared with Fully Convolutional Networks(FCN), SegNet, U-net and DeepLab_V3 in terms of average exchange ratio, similarity coefficient and running time, and the results showed that the proposed RespathU-net achieved good multi-size feature recognition. The effect is much better than other semantic segmentation networks, and we verify the actual defect detection accuracy of the designed network as 87.01% through experiments.