Separators has directly affected the safety and electrochemical performance of lithium-ion batteries. In this study, an alkali etched enhanced polyimide (PI)/polyacrylonitrile (PAN)@ (Cellulose acetate) CA/PI three-layer composite separator is prepared using electrospinning, non-solvent phase separation, and alkali etching methods. The effects of alkali etching on the mechanical strength, thermal stability, and electrochemical performance of the PI/PAN@CA/PI separator are explored. The obtained separator has two different pore structures, and the surface of the alkali etched separator has abundant polar groups, further enhancing the migration rate of lithium-ions. The mechanical strength and thermal performance decrease with the prolongation of alkali etching time. When the alkali etching time is 3 min, the PI/PAN@CA/PI separator has the best comprehensive performance, with a mechanical strength of 17.8 MPa, ion conductivity of 1.22 mS cm^− 1, and interface impedance of 152 Ω. After 100 cycles of charging and discharging at a current density of 1 C, the capacity retention rate is 95.3%. At a current density of 5 C, the specific capacity of charging and discharging can reach 114 mAh g^− 1, which is better than the 87.3 mAh g^− 1 of the initial PI/PAN@CA/PI separator.