The invention discloses lamb fattening TMR (Total Mixed Ration) fermented feed and a preparation method thereof. The feed is prepared from the following raw materials in parts by weight: 27 to 28.4 parts of corns, 4 to 9 parts of wheat bran, 11.5 to 14.9 parts of soybean meal, 10 to 20 parts of silage rice straws, 30 to 40 parts of silage corn straws, 1.1 to 1.4 parts of calcium hydrogen phosphate, 0.2 to 0.3 part of mountain flour, 0.5 part of salt and 0.6 parts of a premix, wherein the premix comprises microelements, vitamins, lysine and a carrier; the microelements comprise Cu, Fe, Mn, Zn, Se and Co. According to the lamb fattening TMR fermented feed and the preparation method thereof, provided by the invention, the silage rice straws and the silage corn straws are used as main rough feed and are matched with grain feed and the premix; after sufficient mixing, a mixture is sealed and fermented to obtain total mixed ration with balanced nutrients. The waste of feed, caused by the fact that lambs avoid eating certain foods, is avoided, and the inheritance potential of the growth of the lambs can be sufficiently expressed.