Based on the HNC-8 system, the online measurement for typical geometrycharacterized parts was realized by using G code macroprogram and secondary development method.The framework of the online measurement system was planned, the software and hardware structure of the online measuring system of numerical control machine tool were built.The connection between the measurement system and numerical control system was realized.The high speed data acquisition I/O module based on STM 32 was used to get the precise position of coordinate axis at the trigger time of the probe.Aimed at the error of the online measurement system, the repeated accuracy was analyzed through experiments. An optimal measurement speed was found out which ensured the measurement accuracy and efficiency when measuring in x, y and z directions. The measurement path was planned and the related macro-program was developed.The basic function of position measurement, cutter offset compensation and the automatic correction for the workpiece coordinate system were achieved. It lays a foundation for the measurement of complex parts and on-line measurement with higher precision and efficiency.