The actual load-bearing capacities of Power Transmission Towers(PTTs) should be estimated by a real-scale model experiment due to their complex structures, therefore an optical full-field 3D deformation measurement method is proposed to overcome the difficulties of large-scale, large-displacement, 3D deformation, rapid multi-point measurement in PTT real-scale model experiments. Based on close-range photogrammetry technology, this method utilizes specialized markers as measured targets to reconstruct the 3D coordinates of those pre-pasted artificial targets through analyzing the captured photo-set in each epoch, then it conducts the coordinate system global registration according to un-movable targets. By tracking and comparing the 3D coordinates of the deformable targets between different epoches, the 3D load-deformation diagram of the PPT stucture is obtained. Acceptance experiment results indicate that the accuracy of this method could reach about 0.1 mm/4 m and real-scale model experiments show the proposed method could meet accuracy and efficiency requirements of PPT load-deformation measurement. Comparing with traditional deformation sensors, the proposed method can satisfy measuring requirements of non-contact(non-interfere), on-spot, high precision, rapid speed, strong anti-jamming and stabilization.