The invention provides a method for on-line assessment of an insulation on-line monitoring system of a substation, which comprises the following steps: (1) carrying out frequency conversion on a power supply signal which is outputted by the on-line monitoring system and has the frequency of M Hz through a frequency conversion circuit and producing a current signal with the frequency of N Hz, wherein N is not equal to M; (2) outputting the current signal with the frequency of N Hz to a plurality of series-connected loops consisting of current sensors and a reference current sensor of the on-line monitoring system in a through manner; and (3) comparing the phase of the current signal which is outputted by the current sensor and has the frequency of N Hz with the phase of the current signal which is outputted by the reference current sensor of the on-line monitoring system and has the frequency of N Hz in the same series-connected loop. The method for on-line assessment of the insulationon-line monitoring system of the substation can safely assess working situations of capacitive equipment and the insulation on-line monitoring system of a lightning arrester under the working situation of the substation.