A water-cooling heat exchanger and a heat-dissipating device are mounted on a heat-generating element of a computer for rapidly guiding and dissipating the heat generated by the heat-generating element. A water-cooling head, a pump and a heat-dissipating fan are stacked up, and an annular heat exchanger is used to surround the above components, thereby to reduce the space of the whole construction to a lowest extent. With the heat-dissipating fan provided in the center of the annular heat exchanger, the airflow generated by the heat-dissipating fan can blow the heat exchanger in multiple directions, thereby to obtain the optimal effect of heat dissipation.