Torrefaction is an efficient way for the biomass dewatering and upgrading before its thermal conversion and utilization. In this work, a gas-pressurized (GP) torrefaction method was proposed to torrefy the biomass wastes. The effect of gas pressure on biomass torrefaction was studied and the reaction mechanism was investigated in detail. The results indicated that the GP torrefied biomass had lower volatile matter content and higher carbon content than the torrefied biomass obtained by traditional method with carrier gas under atmospheric pressure (AP). The high heating values of GP torrefied rice straw and sawdust at 250 degrees C were as high as 17.9 MJ/Kg and 24.5 MJ/Kg, much higher than those of AP torrefied rice straw and sawdust which were13.6 MJ/Kg and 19.2 MJ/kg, respectively. The GP torrefaction at 250 degrees C removed as high as 77.88% and 66.68% of oxygen in rice straw and sawdust, respectively. Furthermore, it converted part of the volatile matter into fixed carbon through promoting aromatization reactions. The GP torrefied biomasses had richer pores and higher specific surface area than the AP torrefied biomasses. This can increase the reactivity for subsequent thermal conversion of the torrefied biomass. It was also found that the gas pressure significantly promoted the thermal decomposition of the hemicellulose and cellulose. The main difference of the reaction mechanism between the GP torrefaction and AP torrefaction was the secondary reactions between the volatiles and biomass. It was because the volatiles were not removed timely during GP torrefaction. Furthermore, the pressure was essential for effectively promoting the secondary reactions.