Cable and prestressed steel bars are important components in bridge structures, and timely monitoring and monitoring of their stress state during construction and operation is extremely important. A magnetic flux sensor based on the principle of magnetoelastic effect measurement can effectively measure the stress of cables and prestressed tendons, and is a new non-destructive testing technology. The principle and performance of magnetic flux sensors were studied in detail in the article, including measurement formulas, temperature effects, the influence of cable components themselves, and sealing and waterproofing performance. When using magnetic flux sensors, it is not necessary to understand the loading history and usage of the components. By sensing the changes in magnetic properties of the components, stress can be measured, which can be used for newly made bridge cables, existing bridge cables, and prestressed tendons. For existing bridge cables, on-site winding of sensors is required. The article introduces the application of magnetic flux sensors in practical engineering projects such as monitoring the stay cables of Zhanjiang Bay Bridge, monitoring the internal prestressing force of Nanning Huludang Bridge, and monitoring the construction and operation of stay cables of Yibin Yangtze River Bridge