The invention discloses a method for collectively removing NOx and mercury (Hg) in coal-fired flue gas by utilizing the secondary combustion of biomass charcoal, which is characterized in that: on the basis of the secondary combustion denitrification and the recycling of flue gas, the biomass charcoal is used as secondary-combustion fuel, the characteristics of the biomass charcoal such as low sulfur, low nitrogen, chloride containing, high ash-coke activity, high energy density, zero carbon dioxide (CO2) net emission and the like are utilized, and the biomass charcoal is sent into a secondary-combustion area of a hearth, the combined removal of multiple pollutants such as NOx, Hg and the like in the coal-fired flue gas can be realized through the catalyst reduction effect of anoxic combustion and biomass charcoal as well as the catalyst oxidation effect of chloride contained flue gas and high-activity ash and coke. The invention also discloses a system for realizing the method, which comprises a biomass charcoal preparing system, a coal pulverizing system, a hearth, a combustion system of the hearth, a flue gas recycling system and a boiler tail-gas processing system. The biomass charcoal is used as the secondary-combustion fuel, so the technical advantages of the secondary combustion of the fuel and the recycling of the flue gas can be integrated, and the method and the system have the advantages that: the investment cost and the running cost are low, the denitrification efficiency and the mercury removal efficiency are high, the CO2 net emission can be reduced, and the like.