This paper proposes a new real time error compensation implementation method for CNC machine tools. The proposed method reduces the complexity of compensation system significantly, which takes full advantage of numerical control system. The Compensation Control software is developed based on original Human-Machine-Interface software of Siemens 840D numerical control system. Meanwhile, the compensation controller shares the CPU of with the Man-Machine-Communication module. Due to it hardly needs any external devices except some necessary sensors, the proposed compensation strategy greatly reduces the cost of building a compensation system and the stability of compensation system is enhanced accordingly. Experiments have been conducted and the results show that the proposed method can improve the accuracy of machine tools dramatically.