Stone-plastic composite is a relatively new engineering material that is widely used in the decoration industry. Determination of the optimal machining conditions for this composite is important for the manufacturing industry. This work investigates the cutting quality of stone-plastic composite during helical milling with diamond cutters, focusing on the damage mechanism and surface roughness of the machined surface. Response surface methodology was used to model and establish the relationship between surface roughness and cutting conditions. Analysis of variance was adopted to study the significant contributions of each factor and two-factor interactions on surface roughness. An inclined U-shaped waviness was found on the machined surface, with damage patterns of cracking and pitting mainly located in the peaks of the cutting waves. The optimal combination for cutting was found to be a helical angle of 70°, cutting speed of 51.2 m/s, feed per tooth of 0.26 mm, and cutting depth of 0.72 mm; hence, these parameters are proposed for use in industrial stone-plastic composite machining for optimal cutting quality and machining efficiency.