The invention discloses a growth promoting substrate for raising watermelon seedlings. The substrate is prepared by using Trichoderma harzianum T83 as a functional microorganism. When the obtained watermelon seedlings which treated by using the growth promoting substrate is compared with control treated seedlings, the plant height, fresh weight of the overground part, the fresh weight of the underground part, dry weight of the overground part, dry weight of the underground part, stem diameter, root vigor, the content of chlorophyll and the number of fungi, bacteria and actinomycetes of roots are increased by 66.76%, 38.12%, 70.24%, 66.03%, 74.07%, 34.23%, 43.13%, 54.93%, 111.19 times, 1.63 times and 4.38 times separately. Through the high-efficiency growth promoting substrate, final planting of microorganisms can be conducted at the roots of the watermelon seedlings during watermelon seedling cultivation, not only can the survival rate of the transplanted seedlings and the growth speedof the transplanted seedlings be improved, but also the resistance of the watermelon seedlings to stress can be improved; therefore, the substrate has a good application prospect.