The shield tail clearance is difficult to be accurately measured in real time during the on-site construction. To address this issue, this article proposes an automatic shield tail clearance measurement method based on the linear structured light. Firstly, a linear laser is used to project a narrow line-shaped indicator laser between the shield shell and the tube. The line laser image projected by the line laser between the shield shell and the tube is collected in real time through an industrial camera. Then, the target features are recognized by image processing and the end coordinates of shield tail clearance are extracted according to monocular vision imaging principle and line structured light coordinate measurement algorithm. In this way, the accurate solution of shield tail clearance value is realized. Experimental results show that the repeatability measurement accuracy of the proposed method is better than 0. 3 mm, and the absolute measurement accuracy is better than 1. 7 mm. This method can realize the real-time accurate measurement of the shield tail clearance within the range of 0. 2~ 1. 9 m in the depth direction of the camera, which can meet the application requirements of the urban subway tunnel construction.