The assembly error characters of numerical control (NC) machine tool were studied using geometric model, and the integrated expression of assembly error characters was proposed, which contains periodic and cumulative components. The characters of assembly error in the signals of builtin sensors were simulated, as well as the relationship with the motion precision of feed system. The simulations show that the periodic component does not affect the positioning accuracy but the dynamic precision, and the amplitude of periodic component could increase under the condition of frequency modulation. The cumulative component which converts direction at the cross point with motion direction has negative effects on the positioning accuracy. The signals of builtin sensors of an NC machine tool were analyzed and the main factor that influences precision of feed system was found out. The precision of the tested NC machine tool was improved by modifying the assembly.