Abstract To identify the performance and mechanism of variant molten salts during biomass pyrolysis in molten salt, the thermogravimetric and differential scanning calorimetry (TG-DSC) analyzer, fixed-bed reactor and thermodynamic equilibrium simulation were applied to study the thermal melting characteristics and stability of molten salt as well as the selectivity for biomass pyrolysis products. The KCl-ZnCl2 is appropriate for the preparation of H2-rich gas and the carbon material with abundant mesoporous structure due to its activation effect. At 850 °C, the pyrolysis gas obtained from KCl-ZnCl2 contained 42.22 vol% H2 with the H2/CO ratio reaching 1.69. The carbonates demonstrated excellent improvement for the gas composition of biomass pyrolysis products, with 75.43 vol% and 70.52 vol% syngas (H2 + CO) collected from the Li2CO3-K2CO3 and Li2CO3-Na2CO3-K2CO3 pyrolysis systems at 850 °C respectively. With the presence of carbonates, the bio-oil and char prepared by biomass pyrolysis also achieved better quality. The thermodynamic simulation revealed the shifting (formation of Li2O) of molten salt composition during biomass pyrolysis. The interaction between Li2CO3 and char under high temperature explained the high yield of CO in pyrolysis gas products, also resulted in the consumption of salts and limited the sustainable use of the molten salt pyrolysis system.