In this paper, the catalysis of potassium on oxy-biomass combustion is studied using thermogrametric analysis (TGA). Effect of potassium salt type (KCl, K2CO3, and K2SO4), loading concentration, replacing N2 by CO2, and O2 concentration on the catalysis degree are discussed. The experimental results show that the comparison between TG-DTG curves of straw combustion before and after water washing, in both 20%O2/80%N2 and 20%O2/80%CO2 atmospheres shows that the water-soluble minerals in biomass play a role of promoting devolatilization and accelerating the biomass char-oxidation ; and the replacing of N2 by CO2 inhibits both the devolatilization and the char oxidation during oxy-biomass combustion. For the biomass devolatilization stage, the potassium catalysis degree monotonously increases with the increase of potassium salt loading concentration. The catalysis degree order of the studied potassium salts is K2CO3 > KCl > K2SO4. For the char-oxidation stage, with the increase of loading concentration, the three kinds of potassium salts present inconsistent change tendencies of catalysis degree. In the studied loading concentrations from 0 to 8 wt.%, there is an optimal loading concentration temperatures for KCl and K2CO3, at 3 and 5 wt.%, respectively ; while for K2SO4, the catalysis degree monotonically increases with the potassium loading concentration. For most studied conditions, regardless of the potassium salt type or the loading concentration or the combustion stage, the catalysis degree in the O2/CO2 atmosphere is stronger than that in the O2/N2 atmosphere. The catalysis degree is also affected by the O2 concentrations, and the lowest catalysis degree is generally around 20 vol.% O2 concentration.