Abstract: The prestressed concrete structure is widely used in the construction of highway and railway bridges. Under the influence of shrinkage and creep, load, and temperature, concrete may crack during long-term service, leading to deterioration of service performance such as steel corrosion. The existing methods such as manual inspection and fiber optic sensing have problems with low timeliness and reliability monitoring concrete cracking. Based on the magnetic leakage effect and stress induced magnetization effect, the author developed a weak magnetic sensor for cracking prestressed concrete structures and conducted reliability analysis and engineering applications. The results indicate that the weak magnetic sensor ha good sensitivity and reliability, and the preliminary monitoring results confirmed by the bridge management department. The system can achieve real-time, long-term, and automated monitoring of prestressed concrete cracking.