Tunnel deformation monitoring is essential for tunnel safety assessment and early warning. As vital components of transportation infrastructure, tunnels require continuous and reliable structural health monitoring to ensure the efficiency of traffic systems. Traditional tunnel deformation monitoring methods, such as total stations and settlement gauges, typically rely on discrete point measurements. These approaches struggle to capture continuous, full-field deformation of the entire tunnel cross-section, which is essential for identifying localized damage and non-uniform settlement. To overcome this limitation, we propose a tunnel deformation monitoring method based on line-structured light. By projecting line-structured light with a laser emitter, the proposed method enables full-field deformation measurement of the tunnel cross-section. This approach provides continuous and dense deformation measurement rather than discrete point measurements. Accurate extraction of the line-structured light from captured images is a key step in the proposed method. To address the difficulties caused by intricate backgrounds in tunnel environments, we propose a background segmentation approach based on the Swin Transformer. The proposed model improves the separation of the line-structured light from the surrounding background interference. It remains effective under conditions such as dust, surface texture variation, and uneven illumination. Across 30 measurement trials, the proposed method achieved a mean error of 0.5834 mm, demonstrating sub-millimeter accuracy, as well as good stability and reliability under non-ideal imaging conditions.