The invention discloses a photovoltaic power generating device capable of conducting sea water desalination in the sea, and belongs to the field of sea water desalination. The photovoltaic power generating device is mainly composed of a water storage tank, a reverse osmosis mechanism, a drainage mechanism, an electric power system and the like; the reverse osmosis mechanism is composed of at least three reverse osmosis balls wrapped by filter screens, the reverse osmosis mechanism is arranged at the position 2 meters below the water surface, and the reverse osmosis mechanism is connected with the water storage tank on the water surface through a return pipe; the drainage mechanism is used for discharging desalted water in the water storage tank; the electric power system automatically generates electricity to provide drainage power by means of three photovoltaic power generating modules arranged in an array mode. According to the photovoltaic power generator capable of conducting sea water desalination in the sea, sea water desalination is conducted through reverse osmosis, the desalination efficiency is high, all kinds of energy is converted into electric energy automatically, and the photovoltaic power generating device can be directly and independently used in the sea and is suitable for being developed and utilized on islands.