Abstract: An uncooled MEMS capacitive thermal detector was designed,and its thermal and thermal-structural behaviors were stimulated with ANSYS.The relations between the temperature response and the IR absorption,the displacement response and the IR absorption,and the device performance and the structure dimensions were studied by using ANSYS simulation tools.It is found that both the temperature response and the displacement response increase linearly as the absorbed IR energy increased.With the increasing of the structure dimensions,the temperature sensitivity,the displacement sensitivity and the thermal time constant increased,but theirs transient performances became worse.The displacement sensitivity of the device reaches a maximum value when the thickness ratio of Al to Si₃N₄ films was 0.5~0.6.