NOVELTY - Distillers grains are used for preparation of vegetable seedling organic composite substrate. The composite substrate comprises distillers grains, biogas residue and crop straw at volume ratio of 9:2-4:1-2.    USE - Distillers grains used for preparation of organic composite substrate for vegetable seedlings (claimed).    ADVANTAGE - The composite substrate has excellent moisture retention property and good air permeability, is rich in nutrients, does not contain inorganic substrate, promotes seedling water and fertilizer utilization and absorption, enhances seedling stress resistance for strong seedling emergence, can completely replace peat as substrate, and not only protects resources, but also reduces cultivation cost.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for organic composite substrate preparation by:    (A) collecting distillers grains comprising by-products from white wine and beer brewing, biogas residue obtained after biogas production from livestock and poultry manure, and crop straw comprising corn straw and plant straw;    (B) crushing raw materials from more than 2 cm to less than or equal to 1 cm;    (C) removing coarse material and mechanical impurities, controlling water content of less than or equal to 1 cm raw materials to 55-65%, adding urea to adjust carbon/nitrogen (C/N) ratio to 20:1 to 30:1, adding quicklime to adjust pH to 5.5-8, adding 0.1-0.3 wt.% fermentation agent, and uniformly stirring;    (D) respectively stacking raw materials into heap with bottom width of 2 m and height of 1.2-1.5 m on ventilated bottom support, surface coating with straw having thickness of 2 cm, covering with plastic film layer, and fermenting; turning once when heap temperature is more than 60-65 degrees C; fermenting at 55-65 degrees C for greater than or equal to 5 days, while turning once every 2-5 days; turning when heap temperature is more than 65 degrees C and regulating water content at 55-65%; cooling and turning once every 10 days when heap temperature is less than 55 degrees C; stopping turning when heap and environment temperature difference for continuous 48 hours does not exceed plus minus 5 degrees C, air-drying into 30-50 cm thick heap, and continuously stacking for 20-30 days, while turning once every 10 days;    (E) uniformly mixing fermented raw materials; and    (F) sampling product to determine if quality indices of product comply with pH of 5.5-8, greater than or equal to 40% particle size of 5 mm, bulk density of 0.10-0.40 g/cm3, total porosity of 60-80%, organic material content of greater than or equal to 20%, total nutrient content of greater than or equal to 2 wt.%, and electrical conductivity of greater than or equal to 1000 mu S/cm, in which the organic material content is organic carbon content in dry substrate multiplied by 1.724, and the total nutrient content is the total amount of nitrogen, phosphorus pentoxide and potassium oxide in dry substrate.