A gasoline storage device, which is regulated to prevent gasoline from vaporizing, and is fabricated from a soft gasoline bag made from soft material that has been processed and coated on both sides so as to be resistant against gasoline and the majority of chemical liquids. The soft gasoline bag can be folded into a small size, directly disposed into a steel gasoline tank and allowed to unfold. Expandable hoses are used to respectively connect to the existing gasoline inlet pipe, gasoline-measuring pipe and manhole of the steel gasoline tank. The soft properties of the soft gasoline bag are used to isolate the gasoline stored within the steel gasoline tank from the air, thereby reducing vaporization of gasoline, pollution and wastage.