Biomass is a renewable carbon source for producing chemicals and liquid fuels. Biomass pyrolysis technology has achieved significant advances in replacing fossil fuels for producing high value-added products considering the current energy crisis and increasingly severe environmental pollution. However, the complex components, low calorific value, high oxygen content, and moisture content in biomass limit the further development of this technology. Currently, biomass torrefaction pretreatment, a mild-pyrolysis process, effectively reduces the moisture content and O/C ratio, improves the energy density of biomass, and enhances the quality of pyrolysis products such as bio-oil, bio-char, and bio-gas. This study reviews the effects of torrefaction pretreatment on the properties of biomass and its pyrolysis products. First, the classification and reactor of biomass torrefaction pretreatment is introduced. Then, the effects of torrefaction conditions (e.g., temperature, duration time, atmosphere, and particle size) on the basic properties of torrefied products are discussed. Furthermore, the regulatory mechanism of torrefaction on the products obtained from pyrolysis polygeneration and catalytic fast pyrolysis is elaborated. Finally, the future development directions of this field are discussed.