Using the symmetric fuzzy number and finite element method, the uncertain scope of cutting force is quantitatively calculated. The cutting speed and back engagement of the cutting edge and feed are selected, and the ambiguity function relation of the cutting force and the three influencing factors are decided by combing the measured data of the cutting force. The uncertain region of the cutting force is given according to this function relation. At the same time, the curve showing the change of cutting force following time is simulated using the thermal-mechanical finite element method. In addition, the distributions of non uniform stress field and temperature field in the workpiece, cutter, and scraps are simulated, which are caused by thermal mechanical coupling. The predicted results show that this method can better predict the uncertain scope of cutting force based on the influence of the cutting force, and the predicted results are more consistent with the actual situation.