The invention relates to an automatic pressure balancing device of a normal pressure tank, which comprises the normal pressure tank, a
breathing opening is arranged on the normal pressure tank, and abreathing tube is connected to the
breathing opening. The automatic pressure balancing device is characterized in that a
breathing bag is hung in the normal pressure tank, a thin neck head is arrangedat the top of the
breathing bag, and the thin neck head extends out of the breathing opening, is fixed on the breathing opening and is communicated with the
breathing tube. When the pressure in the tank drops, air outside the tank enters the
breathing bag hung in the normal pressure tank along the
breathing tube under the action of
atmospheric pressure, so as to complement the pressure in the tank; when the liquid level or the temperature of the tank increases, the pressure in the tank rises, the
breathing bag is extruded, and partial air is discharged from the breathing bag along the
breathing tube. Thus, the media inbreathed and discharged by the normal pressure tank are all air, and the air and the oil gas are thoroughly separated, thereby realizing the
zero emission of the oil gas, and solving the problems of oil loss and environmental
pollution; the automatic pressure balancing device of the normal pressure tank can also be used in the production, storage or transportation process of solvents, chemicals and the like.