The invention relates to a pure natural plant growth regulator. The pure natural plant growth regulator is characterized by consisting of 60 to 90 parts of molasses fermentation liquid, 5 to 15 parts of calcium ammonium nitrate, 1 to 5 parts of boric acid, 5 to 15 parts of water and 0.1 to 5 parts of 1.51% brassin alcohol milk powder. The pure natural plant growth regulator is prepared by the steps of adding quantitative 1.51% brassin alcohol milk powder into a reaction kettle on the basis that the spraying concentration of the 1.51% brassin alcohol milk powder is 0.2 to 0.5 PPM; adding the molasses fermentation liquid serving as a main raw material and a carrier for use; adding calcium ammonium nitrate, boric acid and water, wherein the specific adding steps can be performed according to the actual requirement in the sequences of dissolving soluble materials into water, adding the solution into concentrated liquid and heating the concentrated liquid at a low temperature. The pure natural plant growth regulator has high content of active ingredients, is pure and safe, does not contain hormone, has high utilization ratio and high mixing property, has weak acidity after being dissolved into water, and is favorable for effective absorption and utilization of crops.