Abstract: The oil evaporation loss of light oil tank will result in pollution to atmospheric environment and waste of oil resource. The researches on the oil/gas sealed collection system mainly focus on the recycling or eliminating technologies after the collection of oil/gas, but rarely on the composition and safety of the system. In this regard, the hierachy protection structures of oil/gas collection system are designed respectively for toxic light oil tank and non-toxic light oil tank, and porrameters setting of each pressure protection is introduced. For the problem of insufficient safety measures for oil/gas sealed system, a liquid sealing tank technology of alternating pressure is proposed. Specifically, vertical structure is adopted and it is composed of negative pressure liquid sealing chamber at the upper part and positive pressure liquid sealing chamber at the lower part, which are separated by horizontal battle. A vertical circular pipe is at the central axis of the battle to connect both parts. Besides, the structure form of both chambers and key design parameters are presented, which can provide data reference for the engineering design of the sealed collection system for light oil tank.