In this paper, a two-dimensional phased array antenna beamforming system based on mode diversity is demonstrated for the first time. The system uses few-mode long-period fiber gratings to excite different modes, and utilizes few-mode fiber Bragg gratings and 2 × 2 optical switches to control the propagation paths of optical signals, so as to realize the true time delay control of optical signals of different mode channels and complete the two-dimensional scanning of the beam. In order to prove the feasibility of the two-dimensional phased array antenna beamforming system based on mode diversity, we conduct experimental verification and performance testing of the system using optical switches to select the loop structures composed of the optical circulators. The far-field radiation patterns of 2 × 3 phased array antenna system of different frequencies are tested at different beam pointing angles. The experimental results are compared with the simulation results to demonstrate that the beam pointing angles have no squint. The beamforming system based on mode diversity takes modes as independent channels for the transmission of signals, and excites and processes signals of different modes in a single few-mode fiber core, which effectively reduces the volume and complexity of the optically controlled phased array radar system.