Abstract: Due to the urgent need of long-periodic safe running and in-service testing for pressure vessel, pressure piping and large-scale atmospheric tank, the wireless and automatic crawling instrument has been developed based on the principle of magnetic flux leakage testing technique. Some advanced technology such as wireless remote control, adjustable and floating sensor, and wireless local area network (WLAN) data transmission and communication have been utilized in this instrument. The equipment is mainly composed of drive module, sensor module, signal processing module, device of recording signal positions and signal acquisition and analysis software. It can make effective detection and assessment about corrosion defects for ferromagnetic specimens. The instrument has a high detection sensitivity through testing and field application. It can detect man-made defects of ø1.6 mm hole and ø10 mm×20% thickness spherical pit for pipes and plate specimens with 8mm thickness.