A fluidized bed reactor for ammonia laden wastewater includes a column, a plurality of carrier particles, a first settling tank and a fluidizing means. The column defines a fluidizing chamber therein, and the fluidizing means is adapted for introducing the ammonia laden wastewater into the fluidizing chamber and further into the first settling tank. The reactor is further provided with microorganisms including nitrifying bacteria, anammox bacteria and heterotrophic denitrifying bacteria attached to the carrier particles. Nitrification reaction, anammox reaction and heterotrophic denitrification reaction are simultaneously taking place in the fluidizing chamber to transform ammonia into nitrogen by the microorganisms. A method for treating ammonia laden wastewater is also provided. The fluidized bed reactor is advantageous in the fact that its start-up is significantly shortened and it is adapted to efficiently treat thin ammonia laden wastewater.