With the aim of providing scientific guidance for the application of diamond cutting tools to the machining of stone-plastic composite, this work presents results on the influence of tool geometry and cutting parameters on cutting forces and temperature during helical milling of stone–plastic composite with diamond cutters. Four factors—helical angle, spindle speed, feed rate, and cutting depth—were assessed using a response surface method. Mathematical models were developed and identified by verification testing to accurately predict changes in cutting forces and temperature during composite helical milling. Then, the significant contributions of each factor and of two-factor interactions were determined by analysis of variance, and the trends of cutting forces and temperature were studied using response surface methodology. The optimal conditions in terms of low cutting forces and temperature were determined to be a helical angle of 70°, cutting speed of 51.3 m/s, feed per tooth of 0.24 mm, and cutting depth of 0.5 mm. These parameters are proposed for use in the industrial production of stone–plastic composite material to improve machining efficiency.