The invention discloses a bag for storing a battery assembly of a cell, the bag comprises a frame including a recess of a battery assembly whose inner portion is inserted and head portion is temporarily exposed, an upper and a lower flange which are adjacent to un upper and a lower end portion of the recess, and extending portions at an arbitrary side of the recess; when the extending portions are folded at a front portion of the battery assembly and on the upper and the lower flange, and when the extending portions are sealed together with each other and together with the flanges respectively, a front portion and an upper sealing portion are formed, the front portion and the upper sealing portion define a bag shape portion for implanting an electrolyte solution in the battery assembly; and a lower sealing portion is formed when the folded extending portions are sealed in the lower flange. The invention further discloses a method for assembling the bag shape battery. The invention canplace a battery assembly with an elliptic section or a palestra shape when being in a situation without superfluous space, thereby a capacity-volume ratio can be increased.