Crosstalk caused by discontinuities of transmission lines can seriously affect the quality of signal transmission, which leads to the problem of signal integrity. By introducing the principle of impedance test in vector network analyzer, and comparing the open response calibrated measurement result under Smith chart format, port extension compensated Smith chart format, impedance format and phase format, the paper describes the information stated by these three different formats under frequency domain; Using time transform function of the vector network analyzer to evaluate transmission line under time domain, that can get the transmission line impedance and the impedance discontinuity location information. The measurement result of a 3.5 mm transmission line explains how to use the vector network analyzer to evaluate the impedance characteristics of one transmission line under both frequency and time domain; this provides a new method for transmission line test.