The invention relates to a high-density porous support for treating wastewater of a biological fluidized bed and a preparation method thereof, belonging to a sewage treatment technology. The high-density porous support is prepared from fly ash, clay, coal gangue, a pore forming material, a caking agent and water in percentage by weight. Particularly, the high-density porous support is prepared bymixing, tabletting and forming, drying, calcining, grinding and crushing, reaming with inorganic acid or alkaline, drying, impregnating with a 20-35 percent exocellular polysaccharide water solution and drying in shade. The preparation method comprises the following steps of: firstly, uniformly mixing and stirring 10-20 percent of fly ash, 40-50 percent of clay, 5-20 percent of coal gangue and 2-10 percent of pore forming material, feeding 15-20 percent of water and 2-10 percent of caking agent again and stirring again, and ageing for 3-5 days; secondly, tabletting mixture, drying the mixtureat a temperature of 80-100DEG C for 4-5h, transferring the dried mixture into a muffle furnace and calcining at the high temperature of 1000-1100DEG C for 2-4h; cooling, grinding, crushing and screening by using a sieve mesh with 20-40 meshes; and thirdly, feeding the screened granules into the inorganic acid or alkali and reaming, drying and then impregnating with the 20-35 percent exocellular polysaccharide water solution and drying in shade to obtain the high-density porous support.