The measurement accuracy of vector network analyzer (VNA) is greatly improved through the continually improved calibration methods and the performance of calibration kits. However, with the frequency range of VNA increasing, the hardware performance is becoming the key factor affecting measurement accuracy gradually. In this paper, it analyzes the three system errors of singleport vector network analyzer affect the measurement results. These three system errors are made imperfect hardware performance of VNA. They are directional error caused by the directional coupler, the tracking error caused by insertion loss and the coupling of directional coupler, source mismatch error caused by mismatch of signal source. The author analyzed the impact of the mechanism on the measurement results. The conclusion is of guiding significance for improving the performance of VNA.