The invention discloses a 3D printing preparation method for an injection mold. The 3D printing preparation method comprises the following steps that 1, mixed powder is prepared, and then vacuum induction melting is carried out, wherein the mixed powder is prepared from, by weight percent, 0.4% to 0.5% of C, 0.7% to 0.75% of Si, 0.35% to 0.45% of Mn, 14% to 15.6% of Cr, 0.25% to 0.35% of V and the balance Fe; 2, an atomization powder preparing method is adopted for preparing the powder; 3, a three-dimensional model is established; 4, the injection mold is formed; and 5, the formed injection mold is separated from a substrate, and then the injection mold is placed in a muffle furnace for annealing to improve the plasticity and toughness of the injection mold. According to the 3D printing preparation method, cracking of mold steel in the 3D printing process can be effectively restrained. Meanwhile, C is added to make up ingredient changes caused by element evaporation and carbonization of the injection mold under the action of lasers.