Embodiments of this application provide a battery separator, including a polyolefin-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.