Torrefaction is a pre-treatment process which can improve the properties of biomass, such as improving grindability, increasing heating value, and reducing the hygroscopic nature. Potassium is a well-known alkali catalyst in the thermal reactions of biomass. The influence of potassium on the devolatilization behavior was dependent on the amount of loading potassium. The aim of this study is to figure out the effect of potassium on the thermal degradation of biomass, the rubber wood sawdust was adopted as the feedstock, and the biomass sample was crushed and then dried in an atmospheric oven. Impregnation was used to prepare samples with different potassium ion content equably and adequately. Thereafter, the sample was torrefied by in a thermogravimetric analyser (TG) at different temperature (150, 20, 250, and 300 °C). The experimental results show that the addition potassium can significantly change the chemical kinetics, and further enhanced the thermal degradation of biomass during torrefaction.