Effect evaluation is still a difficult issue in the weather modification field. Numerical models can play a greater role in the effect evaluation of cloud seeding. Developing numerical models with some more realistic simulation of cloud seeding process is a workable way. In this paper, a three-dimensional mesoscale seeding model has been improved through coupling a new AgI (silver iodide) nucleation calculation scheme. Artificial ice crystal predictand and its associated microphysical processes have been introduced into the model, and the realistic seeding operation modes of both ground-based rockets and artillery have can be simulated by the model. The artificial rainfall reduction operation of a convective cloud system in North China on September 1, 2019, was simulated using the above seeding model with a horizontal grid spacing of 500m. The seeding effect of rainfall reduction operation was evaluated, and the nucleation characteristics and working mechanism of AgI agents in convective clouds were analyzed. The results showed the following: 1) Cloud seeding had some impact on the radar echo intensity of the target cloud system. More precipitation particles stayed in the upper cloud region because of the cloud seeding operation, which slightly increased the echo intensity of the upper part of the cloud body and weakened the echo intensity of the middle and lower part of the cloud body. However, the natural evolution tendency of radar echoes of the target cloud system has not been changed by cloud seeding. 2) Cloud seeding achieved certain rainfall-reducing effects. A large-area rainfall-reducing zone appeared downstream of the operation area. The total rainfall amount was decreased, and the rainfall intensity was weakened. The greatest local value of rainfall amount reduction was 0.27mm, and the rainfall in the main affected area decreased by 5.1% on average. 3) Condensation-freezing nucleation was the main nucleation mode of the AgI agent, followed by contact-freezing nucleation mode. 4) Cloud seeding operation caused an overseeding effect. The growth of artificial ice crystals was in a dominant position during competition, and the natural growth processes of other hydrometeors in the supercooling cloud area were suppressed. The deposition growth of artificial ice crystals was the main reason for the increase in depletion of water vapor and cloud water, and deposition latent heat release eventually led to the change of vertical airflow velocity in the cloud. 5) The cold cloud precipitation process was the main physical mechanism of the rainfall. Due to the influence of cloud seeding, the weakening of the graupel melting process (Mgr) in the warm layer led to the decrease of the total mass of raindrops, which was the main reason for the weakening of rainfall, while the decrease in the number of raindrops falling into the lower part of the warm layer was another reason for the weakening of rainfall.