Abstract: Aspecial three-section pulse train is chosen as transmission waveform, then an ultr-aprecision ultrasonic ranging device is designed. A simplified envelope extracting algorithm for real-time processing of embedded system is devised, and the ultrasonic time of flight was accurately measured by finding the position of the pit of the echo envelope. The prototype is developed by using SoC based on ARM+FPGA architecture as the processing core. The test results show that typical measurement error is less than 0.1 mm in the range of 500 mm, and the measurement accuracy remains stable in the range of -30~65℃.