Abstract Potassium is a natural catalyst in biomass thermochemical conversion and plays an essential role in plant’s growth. To figure out the catalytic effects of potassium on the thermochemical behaviors of biomass, the pyrolysis, combustion, and torrefaction characteristics of rubber wood are comprehensively studied using a thermogravimetric analyzer where the biomass is impregnated by potassium carbonate (K2CO3) at different concentrations. The impregnated potassium is clearly exhibited in the spectra of Fourier-transform infrared spectroscopy, while X-ray diffraction indicate that the cellulose crystallinity decreases with increasing the potassium concentration which increases the biomass reactivity in pyrolysis. The ignition temperature of the potassium-impregnated biomass is lowered slightly when compared with the raw biomass; alternatively, its burnout temperature is reduced profoundly, suggesting that the addition of potassium into the biomass can substantially intensify its oxidative reactivity. Considering the treated biomass torrefaction, its cellulose decomposition at 250 °C is intensified, rendering a reduction in the solid yield with increasing potassium concentration. With the same mass loss at 200–300 °C, at least 28% of torrefaction time can be saved for the potassium-impregnated biomass. Accordingly, the catalytic effect of potassium on biomass thermochemical conversion is clearly outlined. Moreover, the produced potassium-rich biochar is conducive to the developments of carbon storage, soil amendment, and negative emissions technologies.