A passive horn integrally formed by using liquid silicone rubber, includes a first outer casing, a second outer casing, an inner casing and an iron piece. The first outer casing includes a sealing ring, a folding ring and a vibrating membrane, and the sealing ring, the folding ring and the vibrating membrane are integrally injection molded into the first outer casing. A groove is disposed in the second outer casing, and the first outer casing is disposed in the groove. The back surface of the vibrating membrane is provided with a casing groove, which is sequentially embedded with the inner casing and the iron piece. A preparation method of a passive horn integrally formed by using liquid silicone rubber, under the condition of integrated injection molding, may improve the yield rate, ensure product quality, save assembly time and labor cost, and improve production efficiency.