To reduce the system dividing error of a grinding machine in gear grinding and to improve the machining accuracy of gears, the relationship among the dividing error of grinding machine, gear eccentricity installation, and the pitch deviation was analyzed, and the calculation method for dividing errors of the gear was obtained. By adjusting the grinding machine according to the calculated dividing error, the dividing error of grinding machine and the cumulative pitch deviation of the gear were decreased and the manufacturing accuracy was enhanced. The method was verified by taking the flat-faced wheel grinding machine Y7125 as an example. Firstly, a mathematic model on gear eccentricity installation and profile deviation was established to figured out the amplitude and phase angles of gear eccentricity installation. Then, based on the relationship among gear eccentricity installation, the dividing error of grinding machine and the pitch error, the value of dividing error was obtained. Finally, the indexing plate of grinding machine was adjusted and calibrated according to the computed dividing error. The experiment demonstrates that the dividing error has been reduced from 17.7 μm to 3.3 μm, and the cumulative pitch deviation of the machined gear is lowered from 46.9 μm to 11.5 μm. Furthermore, the accuracy of pitch is up to grade three. The results prove that this method can be used to reduce the dividing error of grinding machine effectively, so as to reduce the cumulative pitch deviation.