Biomass gasification is currently a hot research topic. To achieve a high hydrogen content in the product gas, the gasification feedstock used in this study is air‐dried pine woodchips. Experiments are performed in a downdraft gasifier by varying the operation parameters of the particle size (60 mesh, 80 mesh, 100 mesh), temperature point (700, 750, 800, 850, 900°C), and steam‐to‐biomass mass (S/B) ratio (0, 0.7, 1.4, 2.1, 2.8). The main effects of particle size, temperature, and S/B ratio on the composition of the product gas are analyzed to predict the optimal operation parameters of biomass feedstock. For pine woodchip gasification, the optimal particle size is 80 mesh or 0.17 mm, the preferred temperature is 850°C, and the optimal S/B ratio is 1.4. Although there is an error between the experiment and the simulation, the difference is not significant. The Aspen Plus model can provide guidance for the gasification of pine woodchips and can be extended to the gasification of other kinds of biomass.