The invention relates to a landfill body and a preparation method of the landfill body. The landfill body is directly subjected to landfill treatment in a landfill site. The landfill body is characterized by being prepared from flying ash and polyethylene, wherein the weight of polyethylene accounts for 5-15% of the total weight, and the weight ratio of the landfill body is improved by 100-300% compared with that of the flying ash. The invention further provides a preparation method and device of the landfill body. By using the preparation method for obtaining the landfill body, the problems that flying ash powder is high in hydrophilia, large in surface area, large in occupied area in simple landfill treatment and the like are overcome, waste polyethylene plastic in a waste incineration plant is sufficiently utilized for solving the problems of waste incineration fly ash forming, solidification, stabilization and the like, and meanwhile the multiple purposes of improving flying ash bulk density, improving the forming intensity, sealing and solidifying heavy metal, dioxin and other hazardous substance, increasing the landfill site storage capacity utilization rate, reducing the flying ash treating cost and the like are achieved.