To evaluate the effect of cloud seeding operation for rain suppression during the opening ceremony of the Nanjing 2014 Youth Olympics Games, the weather research and forecasting model was used to simulate the precipitation processes and the cloud seeding operation. The present work is the first part of the study. First, the effects of precipitation simulation using eight microphysics schemes were evaluated through comparison with the observation. Furthermore, we used Thompson and Milbrandt–Yau microphysics schemes to analyze the cloud structures and precipitation formation mechanisms. The results showed that the cloud structures and precipitation mechanisms simulated by the two microphysics schemes were consistent. The precipitation affecting the Nanjing Olympic Sports Center on the opening ceremony day was generated by the weak mixed convective–stratiform cloud system, and the precipitation processes were dominated by ice-phase microphysical processes. The melting of snow was the main source of rainwater, contributing 72% in the Thompson microphysics scheme and 60% in the Milbrandt–Yau microphysics scheme, and the evaporation was the main sink term, which consumed 94% of the rainwater in the Thompson microphysics scheme and 95.6% in the Milbrandt–Yau microphysics scheme.