Abstract: In this paper, a detection model for many apparent defects of shield tunnel lining based on deep learning modular design is proposed to quickly and accurately identify defects such as water seepage, cracking, falling blocks, and mud & sand leaking on the surface of shield tunnel lining. The model is divided into four modules: data loading, network structure, loss function and post-processing, training and evaluation. Combining the detection principle and data set characteristics of SSD (Single Shot MultiBox Detector) and YOOv4 (You Only Look Once), it is proposed to comprehensively evaluate the matching degree between the prior boxes of the model and this dataset by using two indicators: fitness and maximum possible recall rate. Based on the distribution of defect labeling boxes of the dataset, the K-means method is used to cluster and obtain a set of prior boxes with the highest matching degree.The structure of the SSD model is optimized by considering the structural characteristics of the YOLOv4 model. The results show that the optimized model has a detection accuracy of 0.623, which is nearly 70% higher than that of the original SSD model (0.373). The detection speed has been increased from 40 FPS to 50 FPS, fully proving the rationality of the optimized model.