The invention relates to a vacuum tube vacuum-degree on-line monitoring device of a high-voltage power distribution device and a method thereof; in the device, a capacity coupler carries out coupling to an alternating-current field signal, a coupling electric field signal is led out to a signal pick-up circuit by a capacitance potentiometer low-voltage arm, after carrying out rectification filtering processing, the pick-up signal is converted into a frequency signal through a voltage-to-frequency conversion circuit, and the frequency signal is transmitted to a microprocessor unit after being coupled and separated. Software processing and analysis are carried out to the acquired signal of the microprocessor, so as to realize the vacuum-degree on-line monitoring function with 10-2 of resolution ratio, an acousto-optic alarming unit realizes vacuum degree degradation pre-alarming function, and the monitoring system and other monitoring systems are synchronized and integrated by a communication unit, so as to be convenient for centralized processing, analysis and sharing of the data. In the device, by monitoring the change of the vacuum tube shielding case alternating-current potentiometer, the change of the vacuum degree is monitored; the structure is simple, the cost is low, the disadvantage that the temperature of a crystal element of a photoelectric probe such as pockels is overcome, the measurement is reliable, the installation is convenient, therefore, the invention is a real-time monitoring device with complete function.