The invention belongs to plant nutritious soil and a preparation method thereof and particularly relates to flower nutritious soil and a preparation method thereof. The flower nutritious soil contains the components of biomass water-retaining agent, turf, sapropel, black soil, corn straw, perlite, meerschaum, deer feces, fine sand, inorganic fertilizer, and the like and is processed by the steps of raw material selection, crushing, screening, disinfection, mixing, pH regulation and the like. By adopting the method of mixing organic materials and inorganic materials, the flow nutritious soil has the advantages of reasonable and loosened structure, complete nutrients, stable property, easily obtained raw materials, easy processing and low cost. In addition, the biomass water-retaining agent contained in the flow nutritious soil is a novel, multifunctional and natural macromoleclar polymer and has a three-dimensional space reticular structure, not only can absorb water and fertilizer, but also can retain water and fertilizer, and. the water absorbing capacity of the flower nutritious soil is more than 1000 times the weight thereof, a small reservoir of water and fertilizer is formed in the soil, and the water and the fertilizer are slowly released, thereby the flow nutritious soil has the functions of saving both water and fertilizer. The invention can be used for growing seedlings and planning seedlings.