The motion accuracy of a precision reducer mainly depends on the transmission quality of the cycloid-pin gear, which is determined by the grinding accuracy of the cycloidal gear. The poor meshing characteristics caused by grinding errors in the cycloidal gear are the primary reasons for the low motion accuracy and short service life of a precision reducer. This paper proposes a quantitative correction method based on the grinding error of a cycloidal gear. This method allows the error in machine-tool grinding settings to be quickly and accurately corrected based on mathematical analysis and reverse engineering. A mathematical model of the grinding motion could be constructed based on the particularity of the continuous and closed profile of a cycloidal gear, and a cycloidal profile equation with the grinding settings as variables could be derived. A transitive equation for the grinding settings and tooth profile error was constructed using the closed reconstruction and optimal matching technology, and the essential relationship between the error extraction and quantitative correction was clarified. On this basis, a correction model was constructed, the correction strategy was planned, and the quantified correction of the grinding settings was realized using the numerical optimization method. Experimental results demonstrated that the quantitative correction method could quickly and accurately produce correction values for the machine-tool grinding settings. The tooth profile error could effectively be reduced with just one correction and two grinding operations. This study solved the technical problems that occur in the traditional machining process, which can only be corrected qualitatively and not quantitatively, and provided an effective way to improve the machining precision of the key parts of the precision reducer.