The invention discloses a culture compost for growing pleurotus eryngii by polygonum cuspidatum residues and a preparation method thereof. The culture compost is applicable for growing pleurotus eryngii in a liquid strain, and is prepared from polygonum cuspidatum residues, corncob, sawdust, bran, corn flour, bean pulp, lime and gypsum according to a ratio. The preparation method comprises the following steps: A, pre-treatment of a substrate, piling and fermenting polygonum cuspidatum residues and sawdust respectively for a week in advance, pre-wetting corncob by water, standing in air for later use, the corn flour, the bran and the bean pulp are all dry products, and packaging for later use; and B, weighing polygonum cuspidatum residues, sawdust, corncob, corn flour, bean pulp, lime and gypsum according to the ratio, mixing uniformly with water, stirring mechanically for 1h, adjusting the water content to 60-65%, wherein the pH (potential of hydrogen) is 6.5-6.8. By the factorization of the pleurotus eryngii, the production period is shortened greatly, and the cost is reduced; and the polygonum cuspidatum residues are the wastes of polygonum cuspidatum production, the resource is rich, the particle size is medium, the water retention property is high, through using the polygonum cuspidatum residues to grow pleurotus eryngii, the waste is turned into wealth, the waste of resource is reduced, and the economic benefit is improved.