Abstract: In order to achieve high-precision flow measurement,a transit-time ultrasonic flowmeter based on TDC-GP21 was designed.The MCU of MSP430 is applied as the core of system,a high-precision time interval measuring chip TDC-GP21 which can efficiently solve high-precision measurement of the time interval problem in the transit-time ultrasonic flowmeter measurements.The functional modules and working flow of TDC-GP21 is introduced detailed.Besides,an ultrasonic echo signal processing circuit is designed.The hardware circuit and saftware solutions of ultrasonic flowmeter is interpreted.It is proved by experiment that the maximum measurement error of the system in less than ±1%,reaching the requirements of one level accuracy.