Abstract: Objective: Liquid black garlic was prepared from fresh garlic and the effect of Streptococcus thermophilus on the quality of liquid black garlic was studied. Methods: Fresh garlic was inactivated with alliinase at 180 ℃, then broken and added with Streptococcus thermophilus. Using the inactivated garlic without Streptococcus thermophilus (blank) as reference, the effects of Streptococcus thermophilus ratio, different alliinase inactivation time and their coexistence time on the quality of liquid black garlic were studied. Results: The optimal time for inactivating allinase in garlic at high temperature was 9 min, the optimal proportion of Streptococcus thermophilus was 5%, the optimal coexistence time of Streptococcus thermophilus and inactivated crushed garlic was 32 h, and the fermentation time required in a constant temperature and humidity fermentation tank at 80 ℃ was 15 d. All quality indexes reached the fermentation standard of black garlic. Compared with blank black garlic, the total phenol content of liquid black garlic was 2 times, the total acid content was 1.9 times, the reducing sugar content was 1.3 times, the DPPH· clearance and ·OH clearance were 1.5 times and 1.46 times, respectively. Conclusion: Streptococcus thermophilus can promote the fermentation of black garlic, improve its quality indicators, shorten fermentation time and have great application prospects.