A twisted fiber optic sensor based on intensity modulation is described. The principle and structure of the sensor and the choice of steel strand modules are introduced. The sensor is used to determine the tensile strain and distribution. The experimental results show that the change of the transferring light power has an approximate linear relation with the outer force. The intelligent steel strands with this kind of sensor will have a good application in monitoring the concrete crack and deformation distribution in huge structures such as dams and bridges.