Abstract: Effect evaluation remains a difficult issue to resolve in the weather modification field. Numerical models can play a huge role in the effect evaluation of cloud seeding. Thus, a developing numerical model with more realistic simulations of the cloud seeding process is a viable solution to this concern. A three-dimensional mesoscale seeding model was developed in this study by coupling it with a new AgI (silver iodide) nucleation calculation scheme. Artificial ice crystal predictand and its associated microphysical processes were introduced in the model, and the model can thus simulate realistic seeding operation modes of both ground-based rockets and artillery. The artificial rainfall reduction operation of a convective cloud system in North China on 1 September 2019, was simulated using this seeding model with a horizontal grid spacing of 500 m. Subsequently, the seeding effect of rainfall reduction operation was evaluated, as well as the nucleation characteristics and working mechanism of AgI agents in convective clouds were analyzed. The results revealed the following: (1) Cloud seeding impacted the radar echo intensity of the target cloud system. Furthermore, more precipitation particles remained in the upper cloud region because of the cloud seeding operation, slightly increasing the echo intensity of the upper region of the cloud body and weakening the echo intensity of the middle and lower portion of the cloud body. However, the natural evolution tendency of radar echoes of the target cloud system has not been altered by cloud seeding. (2) Moreover, cloud seeding achieved certain rainfall-reducing effects, and a large rainfall-reducing zone formed downstream of the operation area. The total rainfall decreased, and the rainfall intensity weakened. The greatest local value of rainfall amount reduction was 0.27 mm, and the rainfall in the main affected area decreased by 5.1% on average. (3) The AgI agent’s main nucleation mode was condensation-freezing nucleation, followed by contact-freezing nucleation. (4) Cloud seeding operations caused an overseeding effect. During the competition, the growth of artificial ice crystals was dominant, and the natural growth processes of other hydrometeors in the supercooling cloud area were suppressed. The primary reason for the increase in depletion of water vapor and cloud water was the vapor deposition growth of artificial ice crystals, and the release of vapor depositon latent heat eventually led to a change in vertical airflow velocity in the cloud. (5) The cold cloud precipitation process was the key physical mechanism of rainfall. The weakening of the graupel melting process in the warm layer due to the influence of cloud seeding led to a decrease in the total mass of raindrops, which was a major reason for the weakening of rainfall; moreover, the decrease in the number of raindrops falling into the lower part of the warm layer was another reason for the weakening of rainfall.