A fiberoptic crack sensor based on the bending loss principle is designed according to the different requirements of sensor sensitivity and range in different engineering applications. The working principle of the sensor is introduced, and the theoretical relationship between the optical loss and the crack width is derived. The variation law of optical loss under different bending section sizes is studied, and the critical loss size of the fiber in the bending section is determined, and the most sensitive diameter interval is measured. The feasibility of the sensor is verified by simulating the crack open and closure process of the plexiglass plate. Finally, the performance of the sensor is tested by simulating crack experiments under concrete conditions. The experimental results show that the sensor can realize longterm stable monitoring of the crack development process of concrete structures.