The invention relates to an improved structure of a thermopile sensor, which is to employ a membrane to cover a substrate that has a cavity. Besides, a plurality of thermoelectric elements is formed on the membrane extending outwards from the central side of the membrane and is composed of two different materials connected in series. The material of the element can be a composite of metal material and semiconductor material, and an insulation layer partitions the two materials; therefore, the two materials are connected in series through a contact hole. In addition, the contact hole formed at the central side of the membrane is called hot junction, whereas the contact hole formed at the side of the substrate is called cold junction. Moreover, to enhance the sensing performance of the thermopile sensor, a heat-conducting layer is formed at the center of the membrane, and after the heat-conducting layer is covered with another insulation layer, an absorption film is formed. The invention changes the temperature difference distribution by adding in a heat-conducting layer so as to enhance the sensing performance.