The invention discloses a method for preparing a soilless culture substrate from seaweeds. The method comprises the following steps: grinding seaweeds and mixing the obtained powder with alkali liquor, and placing for 5-10 hours so as to obtain an alkali treatment mixture; regulating the pH value of the alkali treatment mixture to 6.5-7.5 by using diluted hydrochloric acid, uniformly mixing with ground straws, spraying mixed bacterium liquid for uniformly mixing, and composting to obtain a fermentation material; uniformly mixing the fermentation material and shell powder, thereby obtaining the soilless culture substrate. According to the method disclosed by the invention, the seaweed resources are fully utilized, the seaweeds and straws treated by the alkali liquor are subjected to cured fermentation in a composting manner, physical and chemical properties of the seaweeds and straws complement with each other, the volume-weight and porosity of the raw materials in the fermentation process are in a proper range, and the dissolved oxygen amount inside the raw materials is promoted, so that the metabolic activity of microbes can be active, the humification process of organic matters is accelerated, the fermentation material is rich in nutrition, the ratio of various nutritional ingredients is moderate, and aeration porosity and water-holding porosity are suitable for soilless culture of indoor green plants.