The firing temperature of ancient bricks determines their physical and mechanical properties. This paper proposes a new method to determine the firing temperature of ancient bricks based on magnetic susceptibility. By reheating ancient bricks, the magnetic susceptibilities of the ancient bricks at different temperatures were measured, and a magnetic susceptibility change-rising temperature figure was thus obtained. The first large deviation in the figure is the firing temperature of the ancient bricks. Using this method, the estimated firing temperatures of clay bricks that were fired at actual temperatures of 700 ℃, 800 ℃ and 900 ℃ were obtained: 690 ℃, 810 ℃ and 870 ℃, respectively. The small difference between the estimated and the actual temperatures proved the feasibility of the proposed method in determining the firing temperatures of bricks. In this paper, the firing temperatures of 10 ancient bricks in Pingyao ancient city were determined using this method. It was found that the firing temperatures were in the range of 650 ℃~690 ℃ and 850 ℃~870 ℃. Based on this, it is speculated that ancient bricks of Pingyao ancient city were heated by burning straw and firewood. The experimental results of this study could provide a theoretical reference for restoration and conservation of ancient buildings.