The invention provides a light water-retaining matrix suitable for greening of low-load reconstructed roofs. The light water-retaining matrix contains 12-30 volume parts of organic fertilizer, 15-45 volume parts of coco coir, 10-30 volume parts of turf, 15-25 volume parts of vermiculite and 10-35 volume parts of zeolite, wherein the content of organic matter is greater than 30%, the total nutrients are 2-5%, the pH is 5.5-7.5, the EC is less than 3.5mS/cm, the water content is 25-45%, the total porosity is 60-75%, the dry bulk density is 0.25-0.5 Mg/m<3>, and the wet bulk density is 0.65-1.05Mg/m<3>. The invention also provides a method for preparing the light water-retaining matrix. The light water-retaining matrix has the advantages of excellent drainage, water-retaining property and fertilizer-retaining property and overall, balanced and quickly-slowly successive nutrients; the materials are easy to obtain; the matrix product is suitable for greening of low-load roofs in cities and plays an active role in promoting improvement of urban ecological environment quality.