The invention discloses a method for preparing water-soluble solid styrene / acrylic resin and application thereof. The method prepares the required resin by simultaneously performing free radical polymerization reaction and esterification reaction on an aromatic vinyl unsaturated monomer, an aliphatic vinyl unsaturated monomer, an unsaturated carboxylic acid monomer, fat alkanol, an initiating agent and an alpha-methyl styrene dimer in a reactor. In the method, the alpha-methyl styrene dimer is added as a chain transfer agent to reduce the branching degree and avoid the generation of gel and the blockage of the reactor; and simultaneously, the added fat alkanol performs esterification reaction with acroleic acid to inhibit the esterification reaction between an alcohol ether solvent and the acroleic acid, thus the viscosity of the reaction mixture is controlled effectively, and the finally prepared resin has narrow molecular weight distribution (Mw / Mn is less than 2.0), low molecular weight (Mw is less than 10,000), few organic volatile matters, and good wetting dispersion of pigment, and can be applied to preparing water color ink, coating or oil polish.