Polyolefin porous film and preparation method therefor, battery separator, and electrochemical device

The present disclosure relates to the technical field of separators for electrochemical devices, and discloses a polyolefin porous film and a preparation method therefor, a battery separator, and an electrochemical device. The polyolefin porous film of the present disclosure has a network fiber structure, and when observed in a 20,000×SEM image, and the polyolefin porous film has the following characteristics: (1) within a circle having a radius of 1000 nm, a fiber orientation result R_MD of fibers having a fiber diameter of greater than 15 nm in a machine direction (MD) satisfies: (R_MD)