The invention relates to a seedbed soil for culturing blanket-like rapeseed seedlings and a preparation method thereof, belonging to the technical field of agriculture. The seedbed soil is prepared through soil preparation, disinfection, mixing of fertilizers and addition of vegetable fiber. According to the invention, 2.5 to 3 g of pure nitrogen, 1.5 to 2 g of P2O5, 1.5 to 2 g of K2O and 0.15 to 0.25 g of borax are blended with seedling soil, 100 to 200 g of a decomposed organic fertilizer is blended, and 50 to 100 g of vegetable fiber is added. When the seedbed soil is used for culturing blanket-like rapeseed seedlings, the phenomenon of local dying of seedlings almost disappears, watering frequency during seedling culturing is reduced, uniformity of taken seedlings during transplanting is higher, the amount of soil on roots is increased, a root damage rate is decreased, seedlings transplanted into a big field are survived 2 to 3 days earlier compared to seedlings cultured by using traditional methods, and bases are laid for improvement of seedling culturing quality and high-yield of large-field production of the blanket-like rapeseed seedlings.