Abstract: In order to evaluate the feasibility of baling silage made from oat and common vetch mixtures in an alpine area on the Qinghai-Tibet Plateau and to provide a technical reference for local forage production, the effects of fermentation interval and additives on the fermentation characteristics and quality of baled silage were assessed in Xiahe County, Gannan prefecture. Oat and common vetch were harvested at the grain filling and flowering stage respectively, baled and ensiled after treated with (1) corn flour (4% of fresh weight ); (2) urea (0.4% of fresh weight); (3) Synlac Dry (0.0002% of fresh weight); (4) Sila-Max 200 (0.00025% of fresh weight); (5) no additive (CK). All treatments had 3 replicates. Bales were sampled at 40, 80 and 120 days after ensiling. Measurements included microbial counts and community analyses were undertaken by plate-culture. The results showed that silage quality was significantly affected by the fermentation interval. The time needed to complete fermentation was increased because of low temperatures in autumn and winter, about 80 days. Adding urea significantly increased CP (crude protein) content, but NH₃-N content was highest in this treatment at 40, 80 and 120 d, 73.7%, 189.6% and 185.3% higher than the control, respectively. The pH fell slowly remaining above 4.2 at 120 d. The quality of silage supplemented with corn flour was superior to that supplemented with urea. Compared with non-biological additives, lactic acid bacteria promoted fermentation, improving the quality of the baled silage. Sila-Max 200 was superior to Synlac Dry, its LAB (lactic acid bacteria) count as 9.9% higher than the control at 40 d; additionally LA (lactic acid) content was by 110.77% and pH significantly (P<0.05) lower. After 80 d of ensiling, the LA content of Sila-Max 200 was still twice that of the control, significantly (P<0.05) higher than that all other treatments and pH was below 4.1. It was concluded that addition of Sila-Max 200 into baled mixed oat and vetch silage increased silage quality after 80 d ensiling.