The invention discloses an automatic seawater distilling device, which utilizes natural energy and is characterized in that the natural energy like atmospheric pressure, earth gravity, molecule Gibbs free energy and solar energy and so on is utilized as power, a reverse U-shaped tubular structure with certain height combines two open desalting system and salt-collecting system to form an automatic seawater distilling system at normal temperature and negative pressure and realize the automatic production of fresh water (distilled water) and salt. The invention has the advantages that: 1. the natural energy is utilized as the power to produce fresh water and salt, which does not discharge any pollutant, thus saving fuel energy and protecting environment; 2. ductwork is highly automatic and needs no manual operation, which is safe for production and can improve labor efficiency. In a word, no energy, raw material and labor investment is needed, only equipment investment at a first time is needed, and then profit can be obtained for a long time. The invention is also applied for a bitter water region or be installed on different large ships to produce fresh water by utilizing the afterheat of the power and the water in rivers, lakes and seas on the way; also the invention can be installed on a residential building to produce drinking water with tap water, which has good ecological effect, economic benefit and social benefit and wide market.