Time domain reflection technology, as a technique that can intuitively reflect changes in transmission line impedance, is widely used in the field of impedance detection. However, there are problems such as inaccurate calibration and low impedance testing accuracy in the process of measuring the characteristic impedance of transmission lines. In response to these issues, this article designs a time-domain measurement and calibration system for characteristic impedance. The system uses the shortest interval estimation algorithm to measure the voltage on the transmission line, achieving the measurement of the characteristic impedance of the transmission line. By measuring four coaxial cables with different lengths and characteristic impedances, the measurement error is controlled within ±1%, meets the requirements of impedance measurement. The actual results show that the system can accurately measure the impedance of transmission lines. This method simplifies the operation process, avoids subjective bias caused by manual selection, and provides a new method for the development of transmission line impedance detection technology.