The invention discloses a light ecological concrete brick and a preparation method thereof. The light ecological concrete brick is formed by that cement, gravel, pearlite or expanded vermiculite, saw dust (or straw dust, paper scrap and the like), mixed microorganism and water are mixed at certain ratio, are shaped by a mould and are naturally hardened, wherein the mixed microorganism comprises six floras, i.e. bacillus, nitrobacteria, photosynthetic bacteria, saccharomycete, lactic acid bacteria and actinomycete. The light ecological concrete brick of the invention has favourable and long-lasting water purification function because of containing multiple microorganisms and can suspend and even float on the water because of containing light and porous pearlite or expanded vermiculite components with strong adsorption force, which is more favourable for the apposition growth of the microorganism and enlarges contact area with water. The invention can be widely applied on purifying various kinds of water, such as rivers, lakes, ditch water, swimming pools, aquariums and the like.