With the expansion of the railway industry and the upgrade of railway speed, the reliability and efficiency of wireless communication systems in high speed railway attract the public attention. The high speed brings about more severe Doppler Effect, the postponement of handover triggering location and more frequent handover. Therefore the influence caused by the high speed decreases the reliability of wireless communication network in high speed railway. In this paper, a scheme based on the train position information and relay power control has been proposed to optimize the system handover performance. When the train is at the handover point, by power control the relay generates an enhanced signal to meet the minimal communication requirement. The results show that the proposed scheme increases the handover success probability effectively, thus improves the reliability and efficiency of wireless communication systems in high speed railway.