The feed axis’ positioning error of NC machine tool is an important index influencing the machining contour accuracy of workpiece. In current direct monitoring methods for NC system, failures of the positioning error monitoring system caused by damage or pollution of feedback element, are quite common reasons resulting in workpiece’ s contour accuracy out of tolerance, which brings big waste for enterprise. A new positioning error monitoring method was proposed by comparing the measurement values from the two measurement system of Siemens 840D. The proposed monitoring method can avoid defective products caused by the failure of the feedback element to cover the shortage of the current direct positioning error monitoring method and effectively improve the reliability of NC system.