Abstract: Pollution from waste plastics has become one of today’s most serious environmental problems, and the recycling of waste plastics is a research hotspot. High-water materials are widely used in mine-filling operations, leak prevention, flame-retardant fire extinguishers, and other related applications due to their advantages of not blocking pipes, ease of pumping, high early strength, and environmental friendliness. These materials are also commonly referred to as high-water quick-setting materials and high-water filling materials. Despite their advantages, high-water materials also have some shortcomings in practical applications such as the need for a large volume of materials and their high engineering costs. Currently, research on waste-doped modified high-water materials has become an important focus in the development of high-water materials. Using this approach, waste can be treated effectively at a reduced cost by the appropriate replacement of materials. The high-water filling material modified with polyethylene plastic (PE) was used as a carrier. The compressive and shear strengths of the modified materials were determined, and the results were compared and analyzed. The results reveal that with increases in the PE powder content, the compressive and shear strengths of the modified high-water material exhibit a decreasing trend. The stress–strain curves of the modified high-water material obviously differ from those of unmodified high-water material. The residual strength of the unmodified material is higher, that of the modified high-water material is generally low, and no shear displacement curve is evident. The addition of PE powder obviously changes the morphology and microstructure of the material. With increasing PE content, the material gradually changes from having a fiber network structure to a flocculated block structure, in which the formation of larger through holes easily occurs between the products. The shear strength of the modified high-water material is significantly lower than the compressive strength, which indicates that the modified high-water filling is not suitable for coal seams with a large inclination.