The embodiment of the invention discloses a preparation method of a lithium battery diaphragm. The method comprises the steps of: coating the surface of a polyimide diaphragm with a modified polyvinylidene fluoride microsphere coating liquid; performing vacuumizing; adsorbing the polyimide diaphragm with a modified polyvinylidene fluoride microsphere coating liquid to the inside of a diaphragm pore; and performing drying. Compared with the prior art, the method applies and adsorbs the modified polyvinylidene fluoride microsphere coating liquid to the inside the pores of the polyimide diaphragm such that when the lithium battery diaphragm is heated to 120 degrees centigrade, the modified polyvinylidene fluoride microspheres are rapidly expanded and most of the pores of the lithium batterydiaphragm are blocked, and thus the lithium battery diaphragm has a port closing function. When the temperature is lowered, the modified polyvinylidene fluoride microspheres in the diaphragm pores arecooled and contracted, and the pores of the lithium battery diaphragm are unblocked, so that the lithium battery diaphragm can be used normally.