High temperature not only reduces the mechanical properties of mortar, but also has a great impact on its durability. The fire test was simulated by heating the mortar to 400 ℃, 500 ℃ and 600 ℃ by high temperature muffle furnace at three different heating rates (5 ℃/min, 10 ℃/min and 15 ℃/min). Throuth a new experimental technique allowing simultaneous measurement of gas permeability and porosity changes under confining pressure, the changes of gas permeability, porosity and mechanical properties of mortar damaged at different temperatures were studied.The main results show that the gas permeability and porosity of mortar increase gradually with the acceleration of heating rate and temperature. Compared with no heating, when heated to 600 ℃ at 15 ℃/min, the gas permeability of mortar increases by 2 orders of magnitude and the porosity increases by 1.77 times. In the process of loading and unloading confining pressure, the gas permeability and porosity of mortar decrease irreversibly. When the heating rate is faster and the heating temperature is higher, the gas permeability and porosity of mortar are more sensitive to confining pressure. The sensitivity of porosity to confining pressure is smaller than that of gas permeability, but the sensitivity of porosity to confining pressure confirms the irreversible closure of cracks and irreversible fracture of pores during loading and unloading confining pressure. When the mortar is heated to 400 ℃, 500 ℃ and 600 ℃ at three heating rates, the compressive strength of the mortar decreases gradually with the acceleration of heating rate and the increase of temperature.