In cable structures cables are very important components. The magnitude of cable pretensioning is a vital parameter in the design of cable structures and a crucial component in controlling the construction of such structures. Developing a new sensor type is an innovative theme in the monitoring and control of cable structures construction. Based on the elastic magnetic characteristics of ferromagnetic material, we introduce a theory of an elastic magnetic method to measure cable force and developed a new flux sensor. Applying this method in the model test of an hyperbolic cable net demonstrates that it can acquire the accurate cable force value conveniently. This study can provide scientific references for structural engineering.