The WRF-NAQPMS (Weather Research and Forecasting- Nested Air Quality Prediction Modeling System) model are used to simulate radioactive fallout of nuclear explosions from the stabilized nuclear cloud. The distribution of particle radioactivity and the radioactive distribution are described by modeling the stabilized nuclear cloud. In addition, a three-dimensional radiation calculation module and a radionuclide calculation module are constructed based on the vertical diffusion theory of radioactive materials and the radionuclide fractionation model. The model is evaluated against the simulation results produced by HPAC (Hazard Prediction and Assessment Capability) system as well as measured data from three previous nuclear tests (Smoky, Zucchini, and Ess) conducted at the Nevada Test Site. The study demonstrates that the simulated ground exposure rate at 1 h after explosion is close to the measured diffusion trend in both pattern and amplitude. The overall simulation performance of the model is superior to that of HPAC as determined by the MOE (Measure of Effectiveness) and NAD (Normalized Absolute Difference) indices quantitatively. The simulated vertical distribution of exposure rate conforms to the theoretical expectations. The feasibility of the fractionation module is validated by the calculated relative external exposure rate as the function of time and the volatile/refractory ratio in fallout at various arrival time. The significance of this study is providing a new model for simulating and predicting radioactive fallout from nuclear explosions.