Abstract: Biomass is a zero-carbon emission renewable resource that is important for mitigating the greenhouse effect and solving the energy crisis. Pressurized combustion has received increasing attention due to its higher efficiency. Soot is inevitably generated during biomass combustion, which, as nanoscale fine particulate matter composed of polycyclic aromatic hydrocarbons (PAHs), not only reduces the combustion efficiency but also negatively affects the environment, human health, and climate change. However, there is a basic gap in the research on the characteristics of soot generation during biomass pressurized combustion. Pyrolysis is the initial stage of combustion. In this study, a pressurized fixed-bed reactor was used to investigate the soot generation characteristics during pressurized pyrolysis of straw. The experimental results show that the soot production from straw pyrolysis increases firstly and then decreases with the increase of pressure, and reaches the peak at 0.3 MPa. H₂ production increased with the increase of pressure, while the C₂H₂ production increases and then decreases, which indicates that the hydrogen abstraction acetylene addition mechanism is the main mechanism of soot generation in straw pyrolysis at elevated pressure. Tar analysis showed that the amount of aromatic hydrocarbons with four rings increases with increasing pressure. XRD analysis shows that the stacking height of soot crystals increases with increasing pressure. The lamellar growth structure of soot was observed by transmission electron microscopy, indicating that the initial PAHs and soot particle core formation are important for the growth of soot. The average particle size distribution pattern of soot is consistent with the soot yield. The results of this paper show that elevated pressure helps to increase the rate of soot synthesis, leading to an increase in soot yield. However, elevated pressure also leads to the inhibition of the release of soot precursors, so that the soot yield decreases when the pressure is elevated to a certain value. This study contributes to a better understanding of the effect of pressure on soot production during pyrolysis and combustion of biomass.