The invention relates to a system for synthesizing silicon nitride by a fluidized bed reactor. The system comprises the fluidized bed reactor, a nitrogen supply unit and a silicon powder supply unit, the fluidized bed reactor comprises a shell, a lower cavity of the shell is used for performing nitridation reaction for nitrogen and silicon powder under the fluidization condition, an air supply opening and a material supply opening are formed in the shell wall of the bottom of the shell, the nitrogen supply unit comprises a heating device for heating the nitrogen, an air outlet of the heating device is connected with the air supply opening, the silicon powder supply unit comprises a feeding barrel and a Venturi charging device, the feeding barrel is used for containing the silicon powder, a discharging port of the feeding barrel is connected with a powder inlet of the Venturi charging device, a discharging port of the Venturi charging device is connected with the material supply opening, and a discharging port in the top of the shell is connected with a cyclone separator for recovering synthesized silicon nitride. According to the system, the silicon powder and the nitrogen react under the fluidization condition, the synthesized Si3N4 cannot aggregate and cake, and continuous production of the Si3N4 can be achieved.