This application relates to a separator, including a substrate and a coating layer provided on at least one surface of the substrate. The coating layer includes organic particles and inorganic particles, the organic particles include first-type organic particles, the inorganic particles form an inorganic particle layer, and the first-type organic particles are embedded into the inorganic particle layer and form bulges on a surface of the inorganic particle layer. A number-based median particle size of the first-type organic particles is ≥12 μm, and a ratio of an average height of the bulges to a thickness of the inorganic particle layer is ≥4. This application further relates to a separator preparation method, a secondary battery containing such separator, and a related battery module, battery pack, and apparatus.