The invention relates to an ultra-thin secondary battery and a preparation method thereof, belonging to the field of electrochemistry. The thickness of the ultra-thin secondary battery designed by the present invention is 0.3-1.0mm; It comprises separator, positive electrode diaphragm, negative electrode diaphragm; direct contact; the diaphragm is composed of a substrate and a conductive layer); the conductive layer is located on one side of the substrate, and the conductive layer is in direct contact with the positive electrode diaphragm. The preparation method is as follows: first, according to the set size, respectively coat the positive electrode slurry and the negative electrode slurry on the front and back surfaces of the separator, dry and roll to obtain the separator with the positive electrode diaphragm and the negative electrode diaphragm; Pressing, so that the positive electrode current collector and the negative electrode current collector are in close contact with the positive electrode diaphragm and the negative electrode diaphragm respectively. Finally, the packaging film is used for packaging according to the designed size, followed by vacuum dehydration, electrolyte injection, sealing, activation, shaping, secondary packaging, and volume separation to obtain an ultra-thin secondary battery.