Stay cables are the key load-bearing components of cable-stayed bridges, and cable force is the most intuitive indicator of cable service performance. However, cable force is constantly changing due to various factors, making it impossible to directly judge the cable service performance from raw cable forces. Due to the vehicle-induced cable force (VICF) is related to the cable stiffness, a spatial distribution of the relative stiffness of multiple cables is constructed through VICFs to propose an early warning method for the service performance of cables. Firstly, a mechanical model of VICF was established based on the concept of cable force influence coefficient, and a sequence of VICF peaks of the same side cables when a single vehicle crosses the bridge was constructed, which can characterize the relative stiffness relationship of multiple cables. A normalized matrix for the stiffness spatial distribution of the cables on upstream and downstream side is formed. Then, an indicator was established to quantify the change in the cable stiffness distribution matrix, and an early warning method for abnormal service performance of the cables was proposed. Next, the numerical model of the cable-stayed bridge was used to verify the influence of different damage severities. Finally, the effectiveness of the method is verified by monitoring data of actual bridge. The results show that the proposed early warning method can accurately detect the abnormal stay cable from multiple stay cables.