The virtual cathode is an important phenomenon in thermionic emission, and it is widely present in various electronic devices and systems such as vacuum tubes, electron guns, high-power microscopes, X-ray tubes, concentrated solar thermionic converters, and emissive probes. Since the virtual cathode can directly affect the performance of these devices, it is of great significance to study the characteristics of the virtual cathode and conduct experimental measurements on it. In our recent research, a one-dimensional model of thermionic emission was established, and the analytical expressions for the potential barrier and the spatial width of the virtual cathode were derived. With the development of virtual cathode theories, measuring the virtual cathode experimentally has become a reality. In this study, based on our one-dimensional theoretical model, the absolute error theory of the virtual cathode is established, and the contributions of different parameters, such as the hot-cathode temperature, the saturated electron emission current, the electron collection current, Dushman constant, and the work function of hot cathodes, to the absolute errors in the virtual cathode measurement are systematically analyzed. The research results show that the main factors affecting the measurement of the virtual cathode potential are closely related to the size of the virtual cathode. When the virtual cathode potential generated by hot-cathodes is strong, the uncertainty of the hot-cathode temperature becomes the main error source, with a probability of about 61% for the potential barrier measurement, but when the virtual cathode is weak, the main factor becomes the uncertainty of the electron current measurement with a probability of about 39%. Besides, when measuring the virtual cathode width, for common hot-cathodes such as oxide (BaO) cathode, tungsten cathode, and molybdenum cathode, the main factors affecting the measurement results are the uncertainties in the hot-cathode temperature and the work function. These uncertainties account for approximately 94%, 96% and 97% of the measurement variability, corresponding to the above three cathodes, respectively. Only when the virtual cathode is very weak, does the uncertainty of the electron current become the main error source for the measurement of the virtual cathode width.