It has important and practical significance to establish a complete set of health monitoring system for massive project structure. Damage detection is one of the core technologies for structural health identification. Although a variety of damage identification methods have been developed, accurately measuring the response of the structural information is the premise of the effectiveness of damage identification. Damage inside the structure cannot be predicted, so it is not advisable for health monitoring by means of strain gauges. We use quasi-distributed long-gage optical fiber Bragg grating based on glass-fiber reinforced composite material package (GFRP-FBG) and combine it with the damage detection application in simply supported steel beam to verify the performance of the quasi-distributed long-gage GFRP-FBG strain sensor. The test results show that using quasi-distributed long-gage GFRP-FBG strain sensor can effectively perform damage fore-warning and damage location, and estimate the damage degree of bending stiffness.