In the process of airplane take-off and landing, airframe noise due to trailing edge flap is one of the main noise sources, noise reduction technology of trailing edge flap has attracted much attention. By numerical simulation of the unsteady flow field of the flap side-edge, the main characteristics of the generation and evolution of the dual vortex structure in the flow field were analyzed. Then a fence structure with the same dimension as the flap thickness was proposed on the side-edge of the flap. The simulation results show that adding fence has little effect on the aerodynamic characteristics. The fence changes the flow field of the flap side-edge and the structure of the side edge vortex, delaying the development of the side-edge vortex system and making the position of the vortex combination and breakdown far away from the suction surface of the flap, so as to reduce the noise. In addition, after adding the fence structure, the noise still has certain dipole directional characteristics, and the dipole axis is perpendicular to the flap chord line. The noise amplitude was reduced by 1dB~8dB in most directions, and the noise reduction effect was mainly embodied in the broadband noise of the mid-to-high frequency band.