The invention provides a lithium battery composite diaphragm and a preparation method thereof, and the preparation method comprises the following steps: S1, dispersing pretreated montmorillonite in a solvent, and uniformly stirring and dispersing to obtain a montmorillonite dispersion liquid; S2, taking upper turbid liquid of the montmorillonite dispersion liquid, putting the upper turbid liquid into a centrifugal tube for centrifugal treatment to obtain montmorillonite nanosheets, putting the montmorillonite nanosheets into an alkaline solution for infiltration, then performing ice-bath ultrasonic treatment to prepare montmorillonite emulsion, and finally performing solid-liquid separation and drying to obtain montmorillonite powder; S3, uniformly mixing the montmorillonite powder with an oil-based binder to form an oil-based coating slurry; and S4, coating the surface of the lithium battery diaphragm with the oil coating slurry, and drying to obtain the lithium battery composite diaphragm. According to the lithium battery composite diaphragm provided by the invention, rapid transmission of lithium ions on the diaphragm is facilitated, the cycle performance and the ionic conductivity of the diaphragm are improved, the preparation process is simple and feasible, the product purity is high, the preparation cost is low, and the lithium battery composite diaphragm is suitable for large-scale industrial production.