Abstract: The effects of combined modification with different modification amounts on the transmission performance of an internal translational cycloidal pinwheel reducer were investigated. Firstly, the meshing clearance between the internal translational cycloidal gear and the pin gear was analyzed by simultaneously considering equidistant and radial modification. Secondly, based on the deformation compatibility theory, numerical calculations were performed to obtain the maximum meshing force, actual number of meshing teeth, and meshing efficiency of the reducer under the same load with different modification amounts. Furthermore, according to the variation law of the number of meshing teeth and the Hertzian contact theory, the time-varying meshing stiffness of the internal translational cycloidal pinwheel meshing system was calculated, and a dynamic model of the reducer was established based on its structural characteristics. A three-dimensional model of the reducer was also established, and dynamic simulations were carried out to verify the reliability of the dynamic model. Finally, through dynamic analysis, the influence of different modification amounts on the maximum transmission error of the reducer was clarified. The results showed that increasing either the equidistant or radial modification amount led to a reduction in the number of meshing teeth, an increase in the maximum meshing force, and a slight improvement in meshing efficiency. However, the variation in modification amount did not have a linear effect on the maximum transmission error, and specific modification values needed to be considered for accurate evaluation.