Abstract: Based on the correlation of the number of the dominating microorganisms in the solid culture of ‘Tibetan mushroom’ and the trend of nutrients and products, mechanisms were explored for the fermentation processing and antibacterial ability of the fermented milk. Enteropathogenic E. coli, Staphylococcus aureus and Salmonella were used as the model pathogens to investigate the antibacterial ability. Pure lactic acid, acetic acid, and ethanol were used to identify the dominate factors contributing to the antibacterial ability of the fermented milk. Results showed that the numbers of all tested microorganisms decreased in the first few hours, might due to the cells diffused from the solid culture to the milk and the slow cell growth rate. Along with the fermentation, numbers of lactic acid bacteria and yeast kept in a relative stable level, but number of acetic acid bacteria showed significant increase and then decrease. During the fermentation, number of lactic acid bacteria kept in the highest level followed by number of yeast. Acetic acid bacteria showed the lowest level in the fermentation. Lactic acid was identified as the main factor contributing to the antibacterial ability of the ‘Tibetan mushroom’ milk. Acetic acid and ethanol showed the ability in intensifying the antibacterial ability of lactic acid when lactic acid content was low, but the intensifying ability lowered when lactic acid at high levels. The antibacterial activity of the ‘Tibetan mushroom’ milk is contributed to the combination of the activity of bacteria and their products.