The invention discloses a sun tracking device which comprises a thermal-expansion and cold-contraction element, a positioning disk and a pulley. The thermal-expansion and cold-contraction element is of a hollow barrel structure, the surface, facing the sun, of the thermal-expansion and cold-contraction element can extend relative to the surface, back against the sun, of the thermal-expansion and cold-contraction element, and therefore the end face of the thermal-expansion and cold-contraction element tilts. The positioning disk is fixedly arranged on the end face of the thermal-expansion and cold-contraction element, can be tilted along with the tilting of the end face of the thermal-expansion and cold-contraction element, and therefore the lowest point of the positioning disk is made to locate on the surface, back against the sun, of the thermal-expansion and cold-contraction element. The pulley is fixedly connected with a connecting piece and is located on the positioning disk, can move along a circular rail, concentrically arranged with the thermal-expansion and cold-contraction element, on the positioning disk along with the changing of the position of the lowest point of the positioning disk, and therefore the pulley can always stay on the lowest point of the positioning disk. The pulley can drive the connecting piece to rotate to enable external equipment which is fixedly connected with the connecting piece and needs to track the sun to rotate, and the tracking of the sun is achieved. The sun tracking device is high in sensitivity and accuracy, unattended operation can be achieved, and the sun tracking device is small in external environment influence, strong in reliability, small in size, simple in structure, and low in cost.