Error compensation implementing strategy of CNC machine tools is a key technique. Basing on handwheel offset function of CNC system, a novel error compensation implementing strategy were proposed, and an error compensator is developed to realize the error compensation implementing on machine tools with Siemens 840D CNC systems, which communicates with CNC system by the DRF offset function and substitutes for electronic handwheel of CNC system. The error compensator's hardware and software systems, in which multithread technique was used to parallel run to avoid CPU's long-playing employment by one task, were developed. Finally, to test the feasibility of the implementing strategy, an experiment was carried out to compensate geometric error of a heavy-duty CNC floor-typed boring and milling machine tool. Experimental results show that error synthetic compensation equipment developed in this paper can perfectly implement error compensation function.