According to Fick's law and principle of mass conservation, mass transfer model for convective airflow drying of alfalfa was established for forecasting moisture distribution inside alfalfa stem. Through boundary condition treatment of diffusion model and combination of experiment results of moisture diffusion inside alfalfa stem, moisture diffusion coefficient was calculated by using the method of numerical simulation. The result showed that determination coefficient on experimental data with simulation results was 0.927, which indicated that simulation had higher accuracy. Moisture diffusion simulation inside alfalfa stem was in accordance with non-steady drying procedure.