Aiming at the problem that the large-scale structure’s surface is large and the vision of camera is limited, a mobile videometrics method based on relay camera is proposed, and the global scanning measurement is carried out for large-scale structures. Reference mark points are acquired by relay camera fixed on the mobile platform. Extrinsic parameters of relay camera are calculated by the camera parameter calibration method, and the relative conversion relationship among mobile platform coordinates at adjoining times is obtained. A measurement camera goes around structure surface and acquires feature points, which is drived by mobile platform. After that, the real-time extrinsic parameters of the measurement camera can be obtained by the conversion relationship among mobile platform coordinates, and the equivalent binocular intersection collinear equation is established to calculate the positions of feature points. The automatic measurement is realized after completing parameter calibration at initial position. Experiments show that the measurement precision of the proposed method is 2.012 mm, and it is suitable for online monitoring large-scale structure with high degree of automation.