Optical Fiber Bragg Grating (OFBG) is now widely accepted as smart sensor due to its advantages of electric-magnetic resistance, small size, distributed sensing, durability, and so on. However, to our great regret, the bare FBG can only stand 3000~5000με, which can not satisfy the need of practical strain monitoring of infrastructures, especially for the damage detection, such as crack and large strain. In this paper, new technique of sensitivity-decreasing of FBG strain sensor has been brought forward and a new kind of FBG-based crack sensor is also developed. The novel FBG-based crack sensor (also named large FBG strain sensor) can detect 100,000με at maximum, just like 20mm crack at the calibration length of 20 centimeter, and the accuracy can reach 0.01 mm. The new kind of crack sensor is proper for high accuracy crack detection for long-term structural health monitoring of infrastructures.