The rail crack monitoring is a hot research topic to ensure the safety of railway foundation components. The ultrasonic guided wave has the advantages of small attenuation, long propagation distance and high detection efficiency in the rail, which can realize longdistance rapid detection of the rail. The traditional ultrasonic guided wave sensors are generally made by the piezoelectric ceramics. Due to their brittleness and hardness, the traditional PZT ceramic sensors are not suitable for long-term online monitoring of the rail. In this article, the Flexible 0 ~ 3 type of PZT/ epoxy piezoelectric composite is prepared and used as an ultrasonic guided wave sensor. The mechanical properties of the flexible piezoelectric composite and its sensing characteristics for guided waves are investigated by experiments. Combined with the finite element simulation analysis and experiment test, the mode of the excited guided waves and the delay arrival time of the reflected wave from cracks in the rail are analyzed. The applicability and effectiveness of the piezoelectric composite sensor for rail crack guided wave signal detection are discussed. Results indicate that the PZT/ epoxy composite sensor has well flexibility and linear sensitivity, which is adaptable to detect the guided wave under ambient temperature between 5℃ and 75℃ . The guided wave detection signals obtained from different sensing paths of the rail are consistent with the finite element simulation signals. Their arrival time of the crack reflection waves are same. The research provides a new type of flexible piezoelectric sensing technology for on-line monitoring of the rail crack.