The invention discloses a method for preparing a branched poly(methyl) acrylic polymer through emulsion polymerization at room temperature, and belongs to the field of functional polymer preparation. The method comprises the following steps: by taking a functional monomer of a reductive group with polymerizable double bonds and an oxidizer as an initiation system, water as a medium, an anionic surfactant as an emulsifier, sodium bicarbonate as a pH value adjusting agent and a (methyl) acrylic as a monomer, performing an in-situ polymerization reaction for 10-60 minutes at the room temperature, thereby successfully obtaining the branched poly(methyl) acrylic polymer, wherein the monomer conversion rate is up to 85% or greater. By adopting the method, no branching agent is needed, the polymerization cost can be greatly lowered due to use of a commercial polymerizable reduction agent monomer, the polymerization system is simple and stable, feasible to operate, gentle in condition, relatively small in environment influence, low in energy consumption, high in reaction velocity, short in reaction time and high in monomer conversion rate, the temperature does not need to be controlled, the obtained branched poly(methyl) acrylic polymer is large in molecular weight and narrow in molecular weight distribution, and the branching degree of the polymer can be regulated and controlled within a relatively wide range. The method has great significances for theoretic study and large-scale application of the branched polymer.