In this study, atmospheric (AP) and gas-pressurized (GP) torrefaction of Phragmites australis (PAS) were carried out in a rotary tube furnace and a gas-pressurized autoclave, respectively, aiming to investigate the effects of torrefaction pressure on the physicochemical properties and combustion characteristics of torrefied PAS. The organic components of liquid products were also analyzed. Compared with AP torrefied PAS, GP torrefied PAS had a higher weight loss, higher heating value (HHV), lignin content, lower ash content, and atomic O/C and H/C ratios. In addition, GP torrefied PAS had better hydrophobicity, representing a stronger storage capability in humid environments. The organic components of liquid products derived from AP torrefaction were mainly aldehydes, ketones, phenols, and furans, while those from GP torrefaction were mainly phenols, acids, ketones, and alcohols. GP torrefied PAS contained a higher combustion stability and better fuel properties than AP torrefied PAS. The color of GP torrefied PAS was significantly darker than that of AP torrefied PAS, and the weight loss, volatile matter (VM) content, and HHV of AP and GP torrefied PAS showed good correlations with the color parameters. This study innovatively carried out torrefaction in a pressurized atmosphere and used color parameters to establish a model to predict the quality of torrefied products; the results could be applied directly to guide the further development of wetland biomass upgrading and products quality controlling.