Embodiments of this application provide a battery separator, including a poly olefin-based porous separator, where the polyolefin-based porous separator includes polyethylene resin, an elongation rate of the polyolefin-based porous separator in an MD direction is greater than 120%, an elongation rate in a TD direction is greater than 120%, and for the polyolefin-based porous separator, crystallinity at a first-time temperature rise of polyethylene that is measured by using a differential scanning calorimeter is less than 65%, crystallinity at a second-time temperature rise is less than 55%, and a difference between the crystallinity at the first-time temperature rise and the crystallinity at the second-time temperature rise is less than 12%. The battery separator features a high elongation rate and a low temperature of closing a pore. When a battery cell is mechanically abused, a risk of separator breakage can be reduced by implementing embodiments of this application. In addition, a separator pore can be closed in advance when the battery cell is in thermal abuse, to improve safety of the battery cell. Embodiments of this application further provide a preparation method for a battery separator, and a battery and a terminal that use the battery separator.