The internal prestressed reinforcement is located in the interior of the prestressed concrete structure, and it is often difficult to inspect the hidden bend of the prestressed concrete structure no matter in the construction or service stage. Due to pipeline friction, anchorage retraction, instantaneous compression and long-term shrinkage and creep of concrete, and relaxation of prestressed tendons, the internal prestressed stress loss is inevitable. After the loss of prestress, it is very important to monitor the prestress and determine the permanent prestress. Magnetic flux sensor is made based on the principle of magnetoelastic effect of ferromagnetic materials. This measurement method directly monitors the state of components, which is a non-destructive and non-contact measurement method, and the sensor is easy to install and has good long-term stability. In this paper, the magnetic flux sensor is applied to the monitoring of internal prestressed tendons. The measurement principle, sensing performance, temperature compensation, relaxation and fatigue of prestressed tendons, test environment and engineering cases of the sensor are studied in detail. The research results show that the magnetic flux sensor is an effective method to monitor internal prestressed tendons.