The invention is applied to the technical field of three-dimensional sensing, and provides a three-dimensional detection system for the surface of a large thin-shell object and a detection method thereof. The detection method comprises that: three groups of sensors project fringes to the surface of an object to be detected in the upper, middle and lower directions of the object to be detected, acquire a deformation fringe graph, acquires phase distribution information, and acquires three-dimensional depth data of each viewing field by combining phase and depth mapping principle; multi-sensor calibration information is matched with the depth data acquired by the three sensors, and multi-angle data is matched to the same coordinate system; and dimensions are acquired and models are compared, namely the measured three-dimensional data is matched with a computer-aided design (CAD) model, distances from all measuring point to the CAD model are calculated, error distribution pseudo-color pictures of the inner side face, outer side face, inner bottom surface and outer bottom surface of the object, and the related dimension of the object, such as the length, width, height, wall thickness and the like are calculated by methods such as ray tracing and the like.