In this paper，a case of negative dielectric loss duringon-site hand-over test of a parallel capacitor for 500 kV circuit breaker is introduced. It is found through on-site inspection and principle analysis that the wood support structure beneath the parallel capacitor is damped and T-type network is formed.，causing the measured dielectric loss to be negative. In order to explore the influence of each parameter of this T-type network on the capacitance and dielectric loss of the tested object，an equivalent circuit analysis is performed on the T-type network. Matlab is used for simulation analysis. It is found that the measurement error of the capacitance and dielectric loss of the tested object is increased with the increase of stray resistance to ground. With the increase of stray capacitance，the error of dielectric loss is increased firstly and then decreased. The error of capacitance is continued to increase，and the influence of stray resistance is opposite to that of stray capacitance. As for the test environment at site，the corresponding specific measures are proposed so to provide experience reference for the on-site testing of the relevant equipment.