The invention relates to a soilless culture substrate and a relative preparation method, wherein the straw type soilless culture substrate uses corn and wheat straws as main materials, and breaks, ferments, matures and mixes corn and wheat straws in a suitable ratio to obtain basic straw substrate which is composited with furnace slag inorganics to obtain the straw type soilless culture substrate. The inventive soilless culture substrate can replace peat in the soilless culture of vegetable and flower, which can protect peat resource, realize the resource utilization of straws and reduce the environment pollution of incineration or waste, with significant economic, ecological and social benefits.