Currently, great attention has been paid to novel fermented butter possessing high viable bacterial counts, unique flavor, and excellent nutritional quality in the market.To develop such a product, Lactococcus garvieae DZ derived from buffalo milk was used as a starter culture, and the effects of fermentation process and viable bacteria back-addition technology on product quality were investigated. The fermentation process of butter was optimized by single-factor and response surface experiments, and the effect of adding 3% fermented cream back on product quality was analyzed under the optimal parameters. The optimal conditions were determined as follows: inoculum size of 3%, temperature of 37°C, and time of 16 h. Under these conditions, the diacetyl content of the obtained butter was 33.93 mg·kg-1, and the sensory score by the 9-point hedonic scale was 7.85. Using the 3% back-addition process, the viable bacterial count of the final product reached 3.05 × 109 CFU/mL, and the sensory score by fuzzy mathematics comprehensive evaluation was 94.6. Analysis by electronic tongue and GC-MS revealed that the sourness (5.80), umami (8.07), and richness (9.66) of the fermented butter were significantly enhanced. The contents of key flavor compounds, 2,3-butanedione (6188.24 μg·kg-1) and 2,3-pentanedione (4660.37 μg·kg-1), were much higher than those in non-fermented and commercial samples, and the levels of esters and lactones were greatly increased. Fatty acid analysis showed that fermentation increased the content of conjugated linoleic acid from 0.55% to 0.77% and α-linolenic acid from 0.85% to 1.03%, while the proportion of palmitic acid decreased from 28.18% to 24.94%. In conclusion, high-quality fermented butter can be prepared using this bacterial strain combined with the optimized process and back-addition technology, which provides a theoretical basis for the development of functional fermented dairy products.