Alfalfa, rice straw and soybean meal with a mass ratio of 8∶1∶1 were mixed, and applied by 0.0(CK), 1.5(U1), 3.0(U2), 4.5 g/kg(U3) of urea respectively for four groups to prepare mixed silage with an aim to investigate the effect of urea on fermentation quality, nutritional quality and aerobic stability. The results showed that the ammonia nitrogen mass fraction and pH value of mixed silage increased significantly(P<0.05) over the urea increase. The V-score of all treatments were above 83, and the fermentation quality of all treatments were excellent. Meanwhile, the urea-treated treatments dry matter loss rate were significantly(P<0.05) lower than that of CK and the mass fraction of acid detergent fiber in U1 was significantly(P<0.05) lower than other treatments with the mass fraction of neutral detergent fiber in U1 lower than that in CK. And the relative feed value of U1 was the highest, which was 213.69. The aerobic stability of U1 was above 8 d. Conclusively, it can be seen that when alfalfa, rice straw and soybean meal are mixed silage at a mass ratio of 8∶1∶1, 1.5 g/kg urea has the best effect.