Magnesium–sulfur (Mg-S) batteries are emerging as a promising alternative to lithium-ion batteries, due to their high energy density and low cost. Unfortunately, current Mg-S batteries typically suffer from the shuttle effect that originates from the dissolution of magnesium polysulfide intermediates, leading to several issues such as rapid capacity fading, large overcharge, severe self-discharge, and potential safety concern. To address these issues, here we harness a copper phosphide (Cu3P) modified separator to realize the adsorption of magnesium polysulfides and catalyzation of the conversion reaction of S and Mg2+ toward stable cycling of Mg-S cells. The bifunctional layer with Cu3P confined in a carbon matrix is coated on a commercial polypropylene membrane to form a porous membrane with high electrolyte wettability and good thermal stability. Density functional theory (DFT) calculations, polysulfide permeability tests, and post-mortem analysis reveal that the catalytic layer can adsorb polysulfides, effectively restraining the shuttle effect and facilitating the reversibility of the Mg-S cells. As a result, the Mg-S cells can achieve a high specific capacity, fast rates (449 mAh g–1 at 0.1 C and 249 mAh g–1 at 1.0 C), and a long cycle life (up to 500 cycles at 0.5 C) and operate even at elevated temperatures.