The invention relates to a soft oil storage device applied in an underwater vehicle, and particularly relates to a soft oil bag of an underwater vehicle. A bag wall is made of a polyurethane material. The bag wall is fixed on the wall of a cabin of the underwater vehicle by three sealed oil nozzle bolts to ensure the sealing of the wall of the cabin, and is fixed in the oil cabin by a plurality of hangers arranged on bag wall hanger holders. The ideal original state of the oil bag is the vacuum state, the bag walls are pasted together, and fuel oil is injected to the oil bag by an oil injection pump through an oil injection pipe and a sealing oil nozzle at an oil injection port. Along with the injection of the fuel oil, the oil bag is gradually filled. If the oil bag is initially provided with a certain amount of air, the air is exhausted by a sealing oil nozzle at an air exhaust port and an air exhaust pipe along with the injection of the fuel oil. When the fuel oil is required to be used, the fuel oil enters into a daily fuel oil tank through a sealing oil nozzle at an oil supply port and an oil supply pipe. The air bag is gradually shriveled along with the consumption of the fuel oil, and water is replenished into the cabin at any time, so that the function of not changing weight and gravity center parameters along with the fuel consumption of the underwater vehicle can be realized.