Provided is a method of preparing a copolymer which includes 1) initiating polymerization by batch-adding an aromatic vinyl-based monomer and a vinyl cyan-based monomer to a reactor; and 2) performing polymerization by continuously adding an aromatic vinyl-based monomer to the reactor at a predetermined rate after the initiation of polymerization, wherein the continuous addition of the aromatic vinyl-based monomer is initiated when a polymerization conversion rate is 10% or less and terminated when a polymerization conversion rate is between 80 and 90%, and step 2) includes a first temperature phase and a second temperature phase, each of which maintains a constant temperature, wherein a temperature of the second temperature phase is higher than that of the first temperature phase, and thereby a copolymer with a small standard deviation for a vinyl cyan-based unit composition and excellent transparency may be prepared.