To master the pollutant emission characteristics of a 350 MW supercritical CFB boiler under mixed combustion of multi-element low-calorific value coal including coal slime and coal gangue, taking a 350 MW supercritical CFB boiler of a power plant as the research object, and based on the measured data of on-site operation, the formation and emission characteristics of SO2, NOx and dust in flue gas were studied against the boiler load rate between 30% and 99%. Additionally, the key operated parameters including boiler operating load, average bed temperature, excess air coefficient and fluidizing air rate on its generation and emission level were investigated. The test results reveal that the flue gas pollutant emissions of the CFB unit can meet the emission standards within the load range of 30%-99% at total discharge outlet. The emission concentration of SO2, NOx basically decreases first and then increases rapidly with the decrease of boiler load. The concentration of dust emission decreases with the decrease of boiler load, and the dust emission concentration decreases with the reduction of boiler load. Among the key operating parameters studied, the boiler bed temperature plays a leading role in the emission level of SO2, NOx. The emission concentration of SO2 and NOx is higher with the lower average bed temperature and the larger excess air coefficient, and the emission concentration is basically positively correlated with the excess air coefficient and fluidization air rate. Considering comprehensively, it is recommended that the average bed temperature of the boiler should be properly controlled above 800 ℃, and the excess air coefficient should be stabilized less than 1.3. The way of flue gas recirculation can be used to reduce the oxygen concentration of the flue gas under low load on the basis of the maintaining the normal fluidization of the material.