Lithium (Li) metal batteries (LMBs) have received extensive research attention in recent years because of their high energy density. However, uncontrollable Li dendrite growth deteriorates the battery life and brings about severe safety hazards. The rational design of battery separators is an effective approach to regulate uniform Li metal deposition towards boosted cycle life and safety of LMBs. Herein, we review the recent research progress concerning this issue, including mechanically strengthened separator fabrication, functional separator construction towards regulated Li ion deposition, and flame-retardant separator design. Moreover, the key issues and prospects of optimal design of separators are clarified for future development. This minireview is expected to bring new insight into developing advanced separators for long-life and safe LMBs.