Six large scale models of conventionally reinforced concrete coupling beams with span/depth ratios ranging from 1.17 to 2.00 were tested under monotonically applied shear loads to study their nonlinear behavior using a newly developed test method that maintained equal rotations at the two ends of the coupling beam specimen and allowed for local deformations at the beam-wall joints. By conducting the tests under displacement control, the post-peak behavior and complete load-deflection curves of the coupling beams were obtained for investigation. It was found that after the appearance of flexural and shear cracks, a deep coupling beam would gradually transform itself from an ordinary beam to a truss composed of diagonal concrete struts and longitudinal and transverse steel reinforcement bars. Moreover, in a deep coupling beam, the local deformations at the beam-wall joints could contribute significantly (up to the order of 50%) to the total deflection of the coupling beam, especially at the post-peak stage. Finally, although a coupling beam failing in shear would have a relatively low ductility ratio of only 5 or even lower, a coupling beam failing in flexure could have a relatively high ductility ratio of 10 or higher.