Self-heating and spontaneous combustion of torrefied biomass are direct hazards related to the production, storage and transport of torrefied biomass. In this study, we investigated the effect of combined air oxidation and water sorption on self-heating of torrefied agricultural residue biomass at 60, 120 and 180 °C cooling temperatures by measuring weight change and heat flow using thermogravimetric analysis and differential scanning calorimetry (TGA/DSC). Our results showed that the effect of water adsorption predominated at 60 °C. When the gas stream was changed from nitrogen to humid nitrogen, the weight of the torrefied biomass increased rapidly associated with heat generation. Conversely, at 180 °C, the effect of air oxidation predominated and the torrefied biomass decreased rapidly when it was exposed to dry air associated with heat generation. The effect of water adsorption decreased with an increase in the cooling temperature whereas the effect of air oxidation increased with an increase in temperature. The higher torrefaction severity increases the self-heating propensity. The air oxidation and water adsorption may lead to the self-heating of torrefied biomass.