In emergency rescue scenarios, unmanned aerial vehicle (UAV) communications with reconfigurable intelligent surfaces (RIS) is a way to enhance the communications link. However, the key challenges lie in the acquisition of channel information and the design of beamforming due to the limited energy storage and high density integrated antenna array. To solve this problem, we propose an energy minimization (EM) based beam training scheme to optimize the energy consumption of the communication system. Specifically, we consider the RIS assisted UAV communication system with emergency rescue. Considering the constraints of RIS phase-shift, system communication rate and energy efficiency, we propose an optimization problem to minimize the system transmission power, and obtain the optimal solution. The simulation results show that the proposed optimization algorithm can ensure the minimum communication requirements and reduce the transmission power of the system.