The invention discloses a rubber seedling cultivating substrate, which is prepared by uniformly mixing coconut bran, agricultural land surface soil, manure and particle water retention agent, wherein a mixture is obtained by uniformly mixing coconut bran, agricultural land surface soil and manure in a volume ratio of 1:0.3-1:0.3-0.8, then uniformly adding the particle water retention agent into the mixture in a weight rate of 1:1000-3:1000, and the rubber seedling cultivating substrate is prepared. The substrate of the invention has advantages of simple technique and low cost, the seedling cultivating substrate is inproved by utilizing nutrient for replacing the prior nutrient substrate (agricultural land surface soil), reduces damage to agricultural land in seedling cultivating process, and resolves infection to seedling quality because of badness of nutrient substrate to a certain extent.