The invention discloses a stretching device for a lithium battery diaphragm. A base, a second guide roller, a preheating box and clamps are included, a cover body is fixed to the top of the base, the second guide roller is arranged in the center of one side of the interior of the cover body, and first guide rollers are arranged at the positions, on the two sides of the second guide roller, in the cover body; and fixing grooves are evenly formed in the two sides of the end, away from the second guide roller, of the top of the base, threaded lead screws are arranged in the fixing grooves correspondingly, the threaded lead screws are sleeved with driving blocks correspondingly, adjusting arms are hinged to the two sides of each air cylinder correspondingly, and the clamps are arranged on the adjusting arms correspondingly. By installing the base, the cover body, the first guide rollers, the second guide roller, the fixing grooves, the threaded lead screws, the driving blocks, hydraulic telescopic rods, the air cylinders, telescopic columns, connecting rods, the adjusting arms and the clamps, the diaphragm can be stretched more comprehensively and thoroughly, the smoothness of the diaphragm is improved, and the stretching quality of the diaphragm is improved.