Thermal runaway is the main factor contributing to the unsafe behaviors of lithium-ion batteries (LIBs) in practical applications. The application of separators for the thermal shutdown has been proven as an effective approach to protecting LIBs from thermal runaway. In this work, we developed a thermal shutdown separator by coating a thin layer of low-density polyethylene microspheres (PM) onto a commercial porous polypropylene (PP) membrane and investigated the thermal response behaviors of the as-prepared PM/PP separator in LIBs. The structural and thermal analysis results revealed that the coated PM layer had a porous structure, which facilitated the occurrence of normal charge-discharge reactions at ambient temperature, although it could melt completely and fuse together within very short time periods: 3 s at 110 °C and 1 s at 120 °C, to block off the pores of the PP substrate, thereby cutting off the ion transportation between the electrodes and interrupting the battery reaction. Consequently, the PM/PP separator exhibits very similar electrochemical performance to that of a conventional separator at ambient temperature. However, it performs a rapid thermal shutdown at an elevated temperature of ∼110 °C, thus controlling the temperature rise and maintaining the cell in a safe status. Due to its synthetic simplicity and low cost, this separator shows promise for possible application in building safe LIBs.