For better understanding the release and transformation mechanisms of sodium compounds during high alkali coal combustion in circulating fluidized bed (CFB) boilers, a sodium migration model of coal combustion was proposed in this work. The model included submodels of sodium species release, chemical reactions, vapor condensation, particle deposition, and shedding. They were coupled in a three-dimensional CFB comprehensive combustion model to predict the sodium fate. Example simulation was conducted on a 30 kW CFB test rig where experimental measurements have been done. The distributions of sodium compounds in gas, ash, and deposits were computed during the high alkali Zhundong coal combustion. Results show that good agreements are achieved between the simulations and the experiments on the hydrodynamics, temperature, gas species, and sodium distributions.