The invention provides 2,4,6-tri(dimethyl phosphate amido)-1,3,5-triazine-tri-melamine salt and a preparation method and an application thereof. The method employs novel processes of dispersing dissolution of an inertia solvent, acidity control, and solvent washing, which solves the problem of easy partial hydrolysis of phosphatide dur to a structure factor. The embodiment result shows that the 2,4,6-tri(dimethyl phosphate amido)-1,3,5-triazine-tri-melamine salt can be successfully prepared by the method of the invention. The product as a fire retardant for being used in materials of PE, PP and PU has excellent flame retardation performance, and the oxygen index is 22.3-28.5%. In addition, with increase of addition of the 2,4,6-tri(dimethyl phosphate amido)-1,3,5-triazine-tri-melamine salt, the oxygen indexes of PE, PP and PU doped with 2,4,6-tri(dimethyl phosphate amido)-1,3,5-triazine-tri-melamine salt during flame retardation are increased.