Abstract: The products broadcast by the power BeiDou space-ground integrated PPP-RTK augmentation service platform are generated in the latest International Terrestrial Reference Frame (ITRF2020) at the observation epoch, and must be transformed into China’s 2000 National Geodetic Coordinate System (CGCS2000) at the user terminal. To achieve high-precision coordinate transformation, this paper investigates velocity-field interpolation in epoch transformation, as well as the processing order and algorithms used for epoch reduction and frame transformation. First, to determine the optimal velocity-field interpolation algorithm, the accuracy and robustness of linear interpolation and inverse distance weighting (IDW) interpolation are evaluated. The effects of two IDW strategies, fixed neighbor count and fixed search radius, on interpolation accuracy and robustness are analyzed, and a recommended algorithm is provided. Second, to address the engineering controversy concerning whether frame transformation or epoch transformation should be performed first, theoretical analysis is provided to show that the two processing paths are mathematically equivalent when the velocity satisfies certain conditions. It is further shown that the practical deviations are mainly caused by inconsistencies in the velocity-field data sources, rather than by the processing order. Furthermore, simulation experiments verify the effectiveness of the proposed coordinate frame transformation algorithm.