Disclosed are a preparation method for manganese-doped red phosphor, a device and a backlight module including the product. The method includes: 1) mixing A2BF6 polycrystalline particles with mill balls; 2) mixing A2BF6 powder obtained after ball-milling with a hydrofluoric acid for secondary crystallization; 3) filtering out solid particles in A2BF6 and hydrofluoric acid solution after the secondary crystallization; 4) performing ion exchange between A2BF6 particles and A2BF6; and 5) filtering out solid particles to obtain a filter cake, and performing drying treatment to obtain manganese-doped red phosphor.