Structural health monitoring (SHM) technology is of great significance in ensuring structural safety, reducing maintenance costs, and extending the service life of structures. Conventional transducers, which are made of brittle material and installed with additional couplant and fixtures, are difficult to be applied in real engineering structures because of their poor reliability, heavy weight, and excessive volume. Flexible transducer technology is the important direction to solve the existing structural health monitoring problems. The flexible transducers and sensing network, made of polymer materials, can be directly fabricated and integrated on the structures of interests by using advanced additive manufacturing technology, with the advantages in high sensitivity and consistency, light weight and low volume, and good flexibility with complex profiles. For applications in ultrasonic waves-based structural health monitoring,the flexible transducers can form large and condense transducer network on structures, possess wave tuning capability, and qualify for multiple forms of monitoring tasks (including active and passive damage detection), therefore providing solid foundation for health monitoring technology in real engineering structures such as aerospace, railway transportation and oil pipelines.