Abstract: The insulation on the rectifying valve side of converter transformer is mainly subjected to composite AC-DC voltages, and the partial discharge (PD) caused by this type of voltage is obviously different from that caused by pure AC or DC voltage. The simulation model based on hydrodynamic drift-diffusion equations and bipolar charge transfer equations can reflect the distribution of electric field strength and charge density, thus the generation and development of PD can be effectively described. In this paper, the experimental results were used to verify the feasibility of the simulation method, then the simulation model in which the paperboard had a depression due to the electrical aging was established and a variety of composite AC-DC voltages were applied successively. Not only the process of creeping discharge formation and development in the depression were studied, but also the effects of changes in factors such as instantaneous voltage, geometric parameters, voltage proportions, and dielectric constants on the PD were explored. The research on the creeping discharges could partially reveal the physical process and influencing factors of PD at the rectifying valve side of converter transformer, which was conducive to the optimization of the insulation structure and the further research on the creeping discharge mechanism.