Based on expiating the applied limitation of current CAD on internal engagement involutes gear pump, its universal optimal model allowed for user freewill modification under UG software was firstly constituted, and a particular opinion of processing discrete or integral design parameters has been raised；secondly the optimal basic structure parameters of this universal optimal model was formed；its solid model and followed relative analyses, emulation, drawing operations were successively created, the process was all-automatic and all-parametric. Finally the correctness and applicability of the methods and opinions have been proved to be valid by the actual examples.