The invention discloses phenolic resin for 3D printing and a preparation method, and relates to the technical field of casting auxiliary high polymer materials. According to the preparation method disclosed by the invention, the A-stage phenolic resin is modified by the modifier, so that reversible crosslinking points in the resin are increased, and the content of phenolic hydroxyl groups is reduced. According to the modified phenolic resin for 3D printing, four hydrogen bonds are introduced into the resole through the modifier, thermal response dynamic crosslinking points are increased, the resin strength is improved, meanwhile, the proportion of hydroxymethyl is reduced, and the storage stability is improved. Cytosine in the modifier can be dynamically dissociated after being heated and quickly form quadruple hydrogen-bond interaction after being cooled, so that the processing problem caused by viscosity rise is prevented while the strength is improved.