Natural hydraulic lime (NHL) is obtained from calcination and digestion of calcined calc-silicate raw materials. Its main components are dicalcium silicate (Ca2SiO4, C2S) and calcium hydroxide (Ca(OH)2, CH). It fits ancient lithology buildings well, and the durability is in line with the requirements of people for the restoration of ancient buildings. Using X ray diffraction and free calcium oxide (f-CaO) test methods, this paper evaluated the NHL calcination process by investigating the mineral phase composition and conversion rate of the products obtained under different cooling methods (fast cooling speed which is about 500 ℃/min compared with cooling in the closed furnace for the cooling speed of nearly 60 ℃/min), firing time (0 min to 180 min) and calcination temperatures (1 000 ℃ to 1 200 ℃) and with calcite power and silicate power. The results show that, when the calcination temperature is lower than 1 150 ℃, changing the cooling method will not cause the crystal phase transformation of C2S during the cooling process. At the same temperature, the content of C2S in the NHL calcined product increases with the increase of the constant temperature time, and its content reaches the maximum value at this temperature in 30 min to 45 min, and the increase of the calcination temperature increases the maximum value. C2S first exists in β-lattice, and gradually changes to γ-C2S over time. With a rapid cooling speed, the calcination temperature of β-C2S is lower than that of γ-C2S. To prepare NHL, the calcination process could be firing at 1 150 ℃ for 45 min, and with the rapid cooling system.