The invention relates to the technical field of building admixtures, specifically to a stabilizer-plasticizer for wet mixed mortar and a preparation method thereof. The preparation method comprises the following steps of: mixing 4-hydroxybutylvinyl polyoxyethylene ether monomer, acrylic acid, methylacryloyloxyethyl succinate and water, and adding hydrogen peroxide; carrying out a reaction, and then sequentially adding an aqueous solution of a reducing agent, an aqueous solution of a cocatalyst, and an aqueous solution composed of acrylic acid, 2-amino-3-p-hydroxyphenylpropionate, 3-dihydromyrcenol hydroxyphenylphosphinyl propionate and a chain transfer agent; and after a reaction is completed, adding polyvinyl alcohol and adjusting a pH value to 7 to 8 to prepare the stabilizer-plasticizer for wet mixed mortar. Compared with the prior art, mortar synthesized by using the stabilizer-plasticizer for wet mixed mortar provided by the invention has the advantages of low consistency loss, good water retention rate, high compressive strength, high long-term stability and the like, so significant progress is made.