Abstract: To consider cables' sag effects of cable-stayed bridges due to earthquakes, the common approach is to model cables as elastic elements by modifying its elastic modulus according to the cable force under dead load. However, this approach ignored the influence of the cable force on the variation of cables' axial stiffness and that cable relaxation might occur under strong earthquake. This paper firstly presented three different formulations of stress-strain relationship for cables. Based on a single-tower cable-stayed bridge, three nonlinear finite element models were then established by applying different stress-strain relationships of cables. The seismic response of cables and cable relaxation were then studied under near-fault ground motions. Analytical results revealed that large varying cable forces were detected under strong earthquakes and some of the cables experienced cable relaxation. After cable relaxation, the minimum cable force of the cables with cable relaxation no longer decreased while the maximum cable force continued to increase. For the whole structure, cable relaxation was restricted to influence local response and limitedly affected the deck displacement and tower force. However, the seismic response of the structure might be underestimated if the changing axial stiffness from a varying cable force was not considered, with the maximum error reaching 20%.