Abstract: The analytic method on forces when the cycloid disk meshing with fixed pins in a cycloid speed reducer, suggested by V. N. Kudliavchev (Soviet scholar, caused larger errors in project calculation as a result of ignoring the influence of modification for tooth profile on the cycloid disk. Based on discussing the reasonable modification manner for tooth profile on the cycloid disk, a better method of analysis on forces and a calculating formula to fit the backlash clench of modification and the practical project condition are presented in this article.