Abstract: Based on ultrasonic nondestructive test, the ultrasonic testing method for the pin of typical component on the catenaries is studied. Then a flaw detector for catenary components is designed. The instrument is based on FPGA and its main components are a bipolar pulser, a -20-60 dB program-controlled amplifier and an A/D convertor with a digitalization rate of 2.5×10⁵ s^-1. The software of detector is capable of recognizing the crack reflection signal automatically. In the study, both artificial crack and simulative fatigue crack were tested to prove the feasibility of detection method and the reliability of instrument. The portable ultrasonic flaw detector for railway catenary components has advantage of high resolution and automation, which can meet requirement of rapid detection in field work.