In a new development, Mg-based additives were introduced to the process of torrefaction of cotton stalk to enhance the deoxygenation effect. The properties of torrefaction products obtained using temperatures in the range 200-350 C and three different types of Mg-based additives (MgO, MgO-K2CO3, and MgO-KNO3-NaNO3) with varied mass ratios (0.5, 1, and 2) were characterized. The yield of solid product significantly declined from 92.02% to 36.95% when the torrefaction temperature rose from 200 to 350 degrees C, while gas and liquid yields increased to 24.90% and 38.15%, respectively. MgO-K2CO3 was the most effective additive because it not only promoted deoxygenation in the solid product (oxygen content decreased by about 43%) but also reduced the loss of hydrogen. CO2 was the main gas component, and production was promoted as the Mg-based additive mass ratio rose from 0.5 to 2. The phenol and ketone content in the liquid product significantly increased, while the acid content decreased. Biomass torrefaction with Mg-based additives not only produced a low oxygen content solid product but also improved the properties of the byproducts (gas and liquid) so that they could be recovered and utilized as fuel or chemicals.