The invention discloses a method for preparing copolymer miniemulsion by stabilizing and modifying a reactive co-stabilizer, belonging to the field of chemical polymer materials. The method comprises the following steps of: (1) preparing a seed miniemulsion; (2) preparing a miniemulsion supplement material or pre-emulsion supplement material or a pure monomer supplement material; (3) carrying outa semicontinuous supplement polymerization reaction: using one of three supplement materials prepared by the step (2) as a supplement material and dropping the supplement material into the seed miniemulsion in the step (1) by adopting a semicontinuous supplement process for a polymerization reaction; and (4) after finishing dropping, completely carrying out the monomer polymerization reaction to obtain a polymer miniemulsion with the solid content of 45-60 percent. By the invention, the ideal copolymer miniemulsion with high solid content and low viscosity can be prepared by using the characteristics of a semicontinuous miniemulsion polymerization process, preparing the seed miniemulsion by adopting the one-step miniemulsion polymerization and adjusting the solid content and the viscosityof a polymer system by respectively adopting a mode of monomer miniemulsion, monomer pre-emulsion and pure monomer supplement materials.