The invention discloses a large double-direction thrust ball bearing with airtightness and an assembling method of the large double-direction thrust ball bearing. The large double-direction thrust ball bearing comprises a first outer ring, a second outer ring, an inner ring and a ferrule, wherein the side wall of one end of the inner ring extends outwards to form a boss, end surfaces of the first outer ring and the second outer ring are attached and fixed together, the boss is matched with the end surfaces of the first outer ring and the second outer ring to form a first roller way and a second roller way, a plurality of steel balls are mounted in the first roller way and the second roller way, the ferrule is nested on the outer side wall of the inner ring, the first outer ring, the second outer ring and the ferrule are all in clearance fit with the inner ring, and a baffle is mounted at one end, close to the first outer ring, of the inner side wall of the inner ring, and is in clearance fit with the first outer ring. The first outer ring and the ferrule of the bearing adopt a split structure and are convenient to assemble, requirements for the precision of connected parts are low, the mounting is more convenient, a 0.5 mm radial clearance exists between the inner ring of the bearing and the first outer ring and between the inner ring of the bearing and the second outer ring, and mounting errors of a main frame are conveniently compensated due to the clearance.