An airbag and an airbag assembly including a gas generator and an airbag. The airbag (10) forms a first end (11) and a second end (12), the first end (11) being a free end of the airbag, and the second end (12) being used for fixing the airbag to a mounting position. The airbag (10) forms a folding portion (13) and a rolling portion (14) in the collapsed stat. The folding portion (13) is formed by folding one part of the airbag in an “N” shape; the rolling portion (14) surrounds a periphery of the folding portion (13), so that the folding portion (13) is surrounded by an inner side of the rolling portion (14).