Ultra-massive MIMO (UM-MIMO) hopes to further expand the antenna scale, and use beamforming technology to greatly increase spatial gain. UM-MIMO will reduce the width of the main lobe of the antenna beam and expand the range of the near-field area, thereby reduce the effective coverage of the beam, making it unable to meet the coverage needs of highly mobile users in intelligent transportation. Therefore, it is necessary to select an appropriate beam width based on different position and motion parameters. To this end, this paper first proposes a beam coverage model that considers propagation attenuation, thereby providing a theoretical model of the beamwidth coverage. Then, a main-lobe synthesis method based on Dolph-Chebyshev (DC) weighting is proposed to obtain the weighting vector of the array under the specified main lobe beam width. Simulation results shows that the proposed method can effectively adjust the beamwidth.