The present invention discloses a floating seedling substrate special for tobacco and a preparation method thereof. The floating seedling substrate consists of the following raw materials in percentage by weight: 30 to 40 percent of decomposed tobacco straw, 25 to 35 percent of carbonized tobacco straw, 15 to 20 percent of decomposed rice straw and 10 to 15 percent of grass carbon. The preparation method comprises the following steps of: preparing various raw materials according to the mixture ratio of the floating seedling substrate special for the tobacco; leaching the carbonized tobacco straw by water with a pH value of between 5.5 and 6.5 for 5 to 10 minutes; drying the leached carbonized tobacco straw until no open water flows outwards; uniformly mixing and stirring the various raw materials; sterilizing the mixture at the high temperature of 250 DEG C for 10 to 15 minutes; and adjusting water content until the water content meet the requirement of a product so as to obtain the product of the invention. The grass carbon in a conventional seedling substrate mixture ratio is replaced by a large amount of tobacco straw, so the problem of ecological environmental damage due to the use of a large amount of non-renewable resources and the problem of soil ecological environmental damage due to natural decay of the large amount of tobacco straw are solved, the large amount of tobacco straw are recycled and waste materials are recycled. Therefore, the floating seedling substrate special for the tobacco has remarkable economic and social benefits.