The invention discloses a light conversion film capable of promoting plant growth and a preparation method thereof, wherein the light conversion film comprises garnet fluorescent powder with the formula of Y(3-x)BaxAl(5-x)SixO12: mCe, nCr, 0 <= x <= 1, m > 0 and n > 0, and a polymer resin or glass with refractive index of 1.4-1.65, wherein the garnet fluorescent powder is uniformly dispersed in the polymer resin or glass. The preparation method comprises the following steps: (1) respectively weighing compound raw materials of yttrium, aluminum, barium, silicon, chromium and cerium according tothe stoichiometric ratio of each chemical composition of an expression, mixing, and grinding to a micron scale; 2) calcining the raw material powder in a reducing atmosphere for 4-5 hours to obtain acalcined product; and 3, grinding the calcined product to a micron order to prepare Ce and Cr-containing fluorescent powder with a garnet structure, mixing the fluorescent powder with polymer resin according to a mass ratio of 1:4-1:1, and preparing the light conversion film according to the existing film preparation process. The fluorescent powder has the advantages that the emission intensity of the fluorescent powder in a red light region is greatly enhanced, the plant light supplementing capability is high, the efficiency is high, and plant growth is promoted.