The invention relates to the technical field of combustion devices for ammonia gas, and provides a fluidized bed combustion device for ammonia gas, a power generation system and a combustion method ofthe ammonia gas. The combustion device comprises fluidized bed combustor shell, a fluidized bed layer, an air distributing plate, a pumping chamber, a combustion-supporting gas inlet, a fuel gas inlet pipe and a flue gas exhaust port; the air distributing plate is wholly mounted at the lower part of the fluidized bed combustor shell, and the fluidized bed layer is arranged on the upper surface ofthe air distributing plate; the pumping chamber is located below the air distributing plate; the combustion-supporting gas inlet is formed in the pumping chamber; the fuel gas inlet pipe is fixed onthe air distributing plate and communicates with an inner cavity of the air distributing plate; and the flue gas exhaust port is formed in the upper part of the fluidized bed combustor shell. According to the designed fluidized bed combustion device, the problems of has high firing energy, high flame propagation velocity, narrow firing concentration range and unstability of ammonia gas combustionare solved thoroughly. Therefore, the power generation system can successfully use the ammonia gas as fuel, and the remarkable carbon dioxide emission reducing effect is realized.