Abstract: The measurement and estimation methods are discussed and implemented to evaluate the safety of the cable of the cable-stayed bridge. First, a method based on the maximum entropy spectral analysis is proposed, which has some advantages in analyzing vibration responses. Second, the theory of the fuzzy cluster is used to evaluate the cable tension. Finally, taking the Second Nanjing Yangtze River Bridge as an example, the application of above methods is illustrated. It shows that the maximum entropy method generates more effective and accurate results than the traditional spectral method. In three different stages, the cable tension has significant difference between the upper cable and the lower cable. Moreover, it is shown that the maximum entropy method is suitable to mesure cable force under running condition and the fuzzy cluster method is capable of evaluating the safety of the long span cable-stayed bridge.