An energy picking-up system by using water of river, lake and sea as low-grade energy source, includes energy collecting device, energy exalting device and radiator in series. The energy collecting device includes a circuit, which is comprised of a heat collector, a low-grade heat exchanger and a diving pump. The heat collector includes a siphon tube, a drain tube and a vacuum equipment. One end of the siphon tube is placed in the heat collecting well that is located near river, lake or sea, and the other end is placed into the water of river, lake or sea. The vacuum equipment is connected with the siphon tube. One end of the drain tube is connected with the outlet of the low-grade heat exchanger, the other end is placed into or onto the water of river, lake or sea. The diving pump is positioned in the heat collecting well, and is connected with the inlet of the low-grade heat exchanger coin. The energy picking-up system has a simple structure, which is easy to install and maintain, and can work normally in any cases.