The invention discloses a three-layer composite lithium battery membrane which consists of a modified PI (Polyimide) electrospinning membrane and a modified PE (Polyethylene) commodity membrane on thebasis of an electrostatic spinning technology, and a preparation method thereof. The lithium battery membrane is obtained in a way that a modified PI nanometer fiber membrane layer, a modified PE membrane layer and a modified PI nanometer fiber membrane layer are subjected to hot rolling, binding and forming, wherein the modified PI nanometer fiber membrane layer, the modified PE membrane layer and the modified PI nanometer fiber membrane layer are arranged in sequence. The three-layer composite lithium battery membrane provided by the invention has the advantages of small water contact angle, high porosity, ionic conductivity, electrochemical stability window, first-time charging specific volume and specific volume retention rate performance. Especially, the composite lithium battery membrane has both low-temperature thermal hole sealing performance and high-temperature thermal ability. Compared with the PP(Polypropylene)-PE compound membrane which is sold on the market and an existing PVDF (Polyvinylidene Fluoride) electrospinning membrane and PI electrospinning membrane, the three-layer composite lithium battery membrane has higher thermal safety and/ or mechanical performanceand is especially suitable to be applied to the high-end lithium battery membranes, including electric vehicles and the like which have higher lithium battery safety requirements.