Abstract: The first 700 MW high-efficiency ultra-supercritical circulating fluidized bed boiler in China is designed to burn lignite in Southwest China. In order to optimize the engineering design and guide the operation of the boiler, the combustion characteristics, ash formation characteristics and pollutant emission characteristics of lignite in Southwest China were studied on a circulating fluidized bed combustion test bench with a thermal power of 3 MW. The results of coal quality analysis show that the total moisture content of the test coal is 26.15 %, the volatile content of the dry ash-free base is 60.32 %, and the low calorific value of the base is 12.16 MJ/kg. It is a lignite with high moisture, high volatile matter and low calorific value. The ignition temperature of the test coal is 320 °C, and the ignition and stable combustion characteristics are good. The influence of different coal particle size and different limestone particle size on ash formation characteristics shows that the test coal is easy to burst during combustion, and the proportion of coarse particles is larger after limestone is added. The bed pressure of each working condition can be maintained at about 8 kPa, and the differential pressure in the dilute phase zone is maintained above 0.4 kPa. The temperature distribution of the flue gas along the height of the furnace is relatively uniform, indicating that the ash circulation in the furnace can be established normally. The combustion efficiency of the test coal is above 97.54 %, and the combustion efficiency is high. The SO₂ emission without limestone is 3362 mg/Nm³, and the self-desulfurization efficiency is 53.63 %. With the increase of Ca/S molar ratio, the desulfurization efficiency increases. Under the condition of oxygen content of 2.69 %, the NO_x emission of the test coal is 134.82 mg/Nm³. With the increase of oxygen content, NO_x emissions increase. For every 1 % increase in oxygen content, NO_x emissions increase by about 33.4 mg/Nm³. With the increase of Ca/S molar ratio, NO_x emissions change little.