After flame-retardant treatment by the two different agents, the thermal behaviors of Lyocell fibers are discussed. In this research, H3PO4 and NaCl reduced the degradation rate and increased the char yield of the Lyocell fibers, and also increased the limiting oxygen index with the char yield increased. After treatment, the integral procedure decomposition temperature and the activation energy of Lyocell fibers are significantly increased by various concentration factors. These phenomena were indicated by the dehydration, rearrangement, formation of carbonyl groups, the evolution of carbon monoxide and dioxide, and carbonaceous residue formation. These effects were indicating the slow pathway of flame retardancy for the Lyocell fibers and are attributed to the two different flame-retardant agent treatments.