The water tree in XLPE distribution network cables is a threat to the power grid. In order to study the location mechanism of water tree based on broadband impedance spectrum, an experiment is carried out to accelerate the water tree, and the changes of cable micro-element parameters in different stages of water tree are measured. Then a finite element model of cable water tree section is established and the experimental measurement results are verified. The optimal window function and incident waveform in the location algorithm are determined to get an optimized location algorithm. After that, the effectiveness of algorithm is verified according to the experimental results. The results show that the growth of water tree will cause the increase of conductance and capacitance. The location method based on broadband impedance spectroscopy is more sensitive to small changes in local capacitance caused by water trees, but not sensitive to changes in conductance. The localization map of capacitance changes is a in typical double peak structure, and the reflection amplitude is proportional to the local capacitance change.