Using additives in the in-furnace control of arsenic emissions is promising for reducing the impact on the downstream selective catalytic reduction system and blocking the spread of arsenic pollutants into the environment. The study quantifies the arsenic adsorption capacity of kaolinite at high temperature and clarifies its fixation pathway with and without the existence of sodium vapor, which is easily adsorbed by kaolinite. Experiments about Al-coordination and acid sites of products, as well as calculations of thermodynamic equilibrium and the adsorption energy based on density functional theory were performed. During separated arsenic adsorption, nearly 40% of trivalent arsenic [As(III)] is oxidized to pentavalent arsenic [As(V)] and bonded to kaolinite, forming an As-O-Al structure. In this respect, the arsenic adsorption capacity of kaolinite is 200 μg g-1, with 24% of arsenic shown to be well-crystallized Al-bound. During the co-adsorption process, 82% of As(III) is oxidized to As(V) and connected to the Al surface of kaolinite, and the O-Na groups bond to As around the As-O-Al structure, thereby forming Na-O-As-O-Al. The arsenic adsorption capacity increased to 878 μg g-1 with well-crystallized Al-bound arsenic accounting for 56%. This study demonstrates the potential for the application of kaolinite as an arsenic adsorbent in the actual furnace.