<p>This paper investigates the behavior of fluidized thermal backfill material (FTBM) when natural stone aggregates and cement are partially replaced with cross-linked polyethylene (XLPE) waste derived from discarded power cable insulation and class F fly ash, respectively. Specifically, FTBM specimens were prepared by using XLPE waste to replace 0, 5, 10, or 15 vol.% of natural stone aggregates and fly ash to replace 0 or 50 wt.% of cement. The research studied both fresh properties including slump, air content, and unit weight and hardened properties such as bulk unit weight, unconfined compressive strength (UCS), and thermal resistivity of the FTBM. The results indicate that incorporating XLPE increases the slump of the fresh FTBM by up to 20%, barely changes the air content of the fresh FTBM, and reduces the bulk unit weight of the hardened FTBM by up to 2.6% when 15 vol.% XLPE is used, which is beneficial for practical applications of the FTBM. However, the UCS decreased by up to 52% when 15 vol.% XLPE was used, due to the weak bonding between XLPE and the binder as indicated by the SEM images. Furthermore, increasing the XLPE content leads to higher thermal resistivity of the hardened FTBM, rising from 58.8 ℃·cm/W at 0 vol.% XLPE to 72.2 ℃·cm/W at 15 vol.% XLPE. In addition, introducing fly ash to the mixtures further enhanced workability, decreased bulk unit weight, reduced UCS, and increased thermal resistivity of the FTBM. Despite the reduction in UCS and the increase in thermal resistivity, the FTBM with up to 10 vol.% XLPE and 50 wt.% fly ash still satisfies the industry requirements for both UCS (≥ 20.7&#xa0;MPa) and thermal resistivity (≤ 75&#xa0;℃·cm/W).</p>