The throughput of high-speed railway train-ground mobile communication system is not only limited by spectrum, but also limited by high mobility, which has become an important bottleneck restricting the development of intelligent high-speed railway. An effective way to improve the system throughput is to use millimeter wave band with wide continuous spectrum and large-scale Multiple-Input Multiple-Output(MIMO) and multi-beamforming technology. However, the multi-beam streams with fixed beam width will result in serious inter-beam interference when the train is running at high speed. An adaptive beamforming and joint transmission scheme based on optimal beam width is proposed in this paper. When the train is close to the Base Station(BS), all beams are activated to realize spatial multiplexing thereafter to improve system capacity and transmission reliability. When the train is far away from the BS, in order to avoid serious interference between beams, more antenna array elements are utilized to form fewer beams to obtain greater beam gain, and beams of the adjacent BS are cooperated to transmit to improve the throughput of the system. Simulation results show that the performance of the proposed scheme is significantly better than that of the existing beamforming and traditional adaptive selection schemes.