On-line monitoring ultra high frequency（UHF）of partial discharge is important for the maintenance of fully enclosed gas insulated switchgear（GIS）

while the performance of the on-line UHF monitoring system depends onthe sensitivity of the system itself and its sensors

as well as the rationality of configuration and installation of the sen-sors on GIS equipment. This study proposed a field checking method based on comparison of the detection ampli-tudes between the calibrating sensor and the standard sensor

and carried out on-site validation of the on-line UHFmonitoring system. The check results were analyzed and compared with the data from laboratory. It is shown that theproposed checking method is effective

the unsatisfactory detection points of the on-line UHF monitoring system is15.5% of total detection points

and 12.2% of the unsatisfactory detection points relates to the low-sensitivity sensors

which indicates that insufficient sensitivity of the sensors results in unsatisfactory partial discharge detection.