The preparation method of large fluoro-rubber O-ring is made by a mold. The mold is a pair of flat plate molds. A longitudinal circular groove corresponding to each other is arranged on the upper and lower two flat planes of the mold to form a straight round hole at the joint of the upper and lower two flat planes. The preparation method is to first make the fluorine rubber through plastic and mixing processing, use the extruder to squeeze the rubber material round strip, put the rubber strip into the cavity groove at the lower part of the mold, and leave a little of the head outside the mold when the first section is fed, close the upper die to pressure, and heat the mold to 155℃ ~ At 165℃, the mold is opened after pressure, the sequence of processing is processed step by step, and the remaining section is not processed, and then the rubber strip is removed, and the part of the tail and the unprocessed head are bonded together in a ring shape, and the vulcanization is carried out again. The product made by this method has the advantages of high precision, reduced production cost, simple operation and high yield.