Abstract: The roasting characteristics of small grain green coffee beans in Yunnan province and the optimization of their roasting process were explored in this paper. The roasting characteristics were discussed by measuring the color，weight loss rate，moisture content， and water activity of coffee beans under different roasting conditions. The results showed that the L* value ranged from 63. 78 to 35. 09. Due to endothermic dehydration，high-pressure bursting，and organic matter conversion，the weight loss rate and moisture content of coffee beans showed a downward trend with the increase of roasting time and temperature，and the temperature increase would accelerate Maillard browning and flavor material generation. The changes of Aw changed the sweet-acid balance and bitter substances of roasted coffee through the Maillard reaction and caramelization reaction，and the condition of Aw < 0. 33 promoted the conversion of the Maillard reaction to the caramelization reaction， thus presenting the caramelized bitterness of deep roasted coffee. Through the design and optimization of univariate experiments and response surface experiments， the optimal roasting process parameters of small grain green coffee beans in Yunnan province were determined as follows：the roasting temperature of 222 ℃，the roasting time of 17 min，the roasting weight of 65 g. The physicochemical properties and flavor quality of coffee beans were the best under the condition，and the composite weighted score composed of weight loss rate，expansion rate，and sensory evaluation was 34. 212. The results provide technical support for improving the coffee bean roasting process， flavor control， and quality improvement of coffee products in China.