Free-form milling is a flexible gear machining method that allows the use of general disc cutters to machine various gear types on a 5-axis machine tool. This paper proposes a two-dimensional simulation method for free-form milling of cylindrical gears with disc cutters. A mathematical model for free-form milling of cylindrical gears with disc cutters is constructed. By analyzing the spatial positional relationships between the cutter and the workpiece, the instantaneous contact line is derived and projected onto the gear end face; then, the intersection of the projected curves is used to obtain the final profile. This calculation method enables rapid simulation of the actual cutting conditions on the gear end face, which helps determine the machining interference and analyze the tooth profile accuracy during the gear machining process. The method proposed in this paper can also be used to analyze the sensitivity of geometric position errors of the machine tool axes.