The measuring method of structural strain by a 3*3 passive-demodulated fiber optic interferometric sensor was developed to implement the real-time monitoring of structural status. A 3*3 fiber optic Michelson interferometric sensor was constructed to sense the value and the direction of structural strain. This sensor was applied on the cantilevered aluminum beam to experiment the sensing of the structural deformation. The digital signal processing was programmed by LabVIEW to determine the structural strain from the fiber optic signals. This program was verified by various simulated fiber optic signals. Finally, the structural was well determined by this developed program from real fiber optic signals.