Fuel variability effects on physicochemical properties such as adiabatic flame temperature and laminar flame speed of premixed bio-syngas combustion are investigated via polynomial chaos expansion (PCE) based uncertainty quantification (UQ) approach at several equivalence ratios. Questions regarding confidence level of using bio-syngas with varying fuel composition are tackled from a statistical point view. Impacts of unburnt gas temperature and different chemical mechanisms (GMI-Mech 3.0 and San Diego Mechanism) on predicted uncertainties of these combustion properties are discussed. It was found that fluctuation of flame temperature at various equivalence ratios is less affected by bio-syngas fuel variabilities, while flame speed is sensitive to uncertainties in fuel composition. For instance, 1.5% fluctuation of bio-syngas constituent can lead to 14% fluctuation of flame speed for rich combustion, and 3% for lean combustion. Less than 0.8% fluctuation of flame temperature due to variability of bio-syngas fuel composition was observed. UQ of bio-syngas combustion showed that hydrogen variability plays a significant role (70-80% at lean condition) in flame speed variation, while methane variability, although thought to be important, has a negligible impact except for fuel-rich combustion. Overall, the current study has provided a fundamental understanding of the effects of fuel variability on physicochemical properties of bio-syngas combustion. Dominating compositions to variations of biosyngas combustion are provided quantitatively to guide targeted uncertainty reduction from the upstream gasification process.