Abstract: To scientifically evaluate the anti-water-treeing performance of insulating materials for cables and clarify the applicability of different test standards, systematic water-tree growth aging tests were conducted on cross-linked polyethylene (XLPE) and polypropylene (PP) insulating materials according to the test methods specified in GB/T 21224—2007 and ASTM D6097: 2016, respectively. Water-tree length, probability of tree initiation, and microscopic morphology were measured and examined to systematically compare the two test methods in terms of acceleration capability, material discrimination, and applicability. The results indicated that the method in ASTM D6097: 2016 significantly promoted water-tree growth in PP insulating materials and effectively distinguished the anti-water-treeing performance among different PP insulating materials. In contrast, the method in GB/T 21224—2007 was more representative of practical engineering conditions and was suitable for XLPE insulating materials, although it exhibited insufficient water-tree induction capability for high-crystallinity PP insulating materials and showed relatively low discrimination sensitivity. Based on the analysis, the applicable standards and the principles for selecting test methods for PP and XLPE insulating materials were clarified. These findings provide a reference for the development of insulating materials and the selection of test standards.