Backfilling mining is an important way to achieve safe and green mining in mines. It requires a wide origin of source of backfilling materials, low cost, and must meet the requirements of the backfilling process. In order to explore the influence of loess content on the fluidity and mechanical properties of paste backfilling materials, the parameters of yield stress and compressive strength of paste backfilling materials under different types of fly ash and different loess content were determined through laboratory experiments. The results show that the backfilling body made of loess instead of fly ash is convenient and low cost, and large particles are well wrapped in loess particle, which reduces the wear on the inner wall of pipeline during particle transportation and improves the fluidity of the slurry. With the increase of loess content, the compressive strength of backfilling body gradually increases, but when the mass fraction of loess exceeds 12.5%, the loess fine particle hinders the bonding between cement and other materials, resulting in a decrease in compressive strength. When the mass ratio of loess to fly ash is 2∶8, 3∶7, and the curing period is 28 d, the backfilling material not only meets the requirements of backfilling process, but also reduces the backfilling cost to the maximum extent.