The reaction of CaO with HCl was investigated in a fixed-bed reactor fabricated of quartz glass at high temperatures (600−800 °C). The inlet HCl concentration was 2000 ppm. Two sorbents with mean particle sizes of 163 and 465 μm were obtained by calcination of CaCO3. The reaction rate constant was achieved by a deactivation model. The results show that sorbent S1 with small particle size has the higher reactivity and is insensitive to the reaction temperature. In this study, two sorbents have the highest capacity for binding HCl at 650 °C. The breakthrough time of sorbent S1 with a small particle size is longer than that of S2 with a large particle size. A quantity of CaCl2 is evaporated at high temperatures, and it is one of main causes that the outlet HCl concentration does not increase sharply due to the formation of the CaCl2 product layer. Possible mechanisms are proposed to explain the decrease of the chemical reaction rate with increasing temperature.