With its excellent theoretical performance (1675 mAh·g^-1 and 2600 Wh·kg^-1 based on S), low cost and environmental friendliness, Li-S batteries have become one of the most promising candidates for next generation rechargeable energy storage devices. However, the severe shuttling of lithium polysulfides results in the decrease of capacity and short life. In order to promote its commercial application, it is the key point to suppress the shuttling effect. The commercial separator has a large pore size up to about 500 nm, and is ineffective in suppressing the migration of soluble lithium polysulfides. Hence, it is an effective strategy to introduce the functional modification layer to the separator. This article reviews the principles for surface modification of separator and the newly developed separator based on the principle. Moreover, the prospect of separator modification in improving Li-S batteries is prospected.