Grounded DC tree is a serious threat for the safety of HVDC cable insulation. In order to investigate the effect of DC pre-stress on initiation characteristics of grounded DC tree in cross-linked polyethylene（XLPE）

we experimentally investigated the grounded DC tree with different pre-stress time（10 s

5 min and 1 h）. Besides

we measured the space charge accumulation characteristics of the material used in the electrical tree tests

and analyzed the grounded DC tree properties by the space charge injection theory and the space charge test results. The grounded DC tree test results show that pre-stress time has little influence on the initial rate of the grounded DC tree. However

the length of the grounded DC tree increases to a certain extent with the pre-stress time and has a trend of saturation. Besides

there is an obvious polarity effect for the grounded DC tree. The 50% tree inception voltage of negative polarity（-19.3 kV） is significantly lower than that of positive polarity（25.7 kV）

and the average tree lengths of negative polarity are larger than those of positive polarity. The results help to further understand the initiation mechanism of DC electrical tree and provide a reference for the parameter selection of later similar experiments.