With the development of communication technology and the boost of intelligent terminals， mobile content consumption has been witnessed a significant increase. While people enjoy the multimedia entertainment experience brought by wireless network anytime and anywhere， it also brings great pressure to the cellular networks， especially the repeated transmissions of popular videos， which leads to the waste of network resources. The caching technology is considered to be an effective way to solve such problems. It stores popular videos in advance at the edge of the network in off-peak hours to relieve the pressure of network traffic during peak hours. To provide users with better content transmission efficiency， the buffer-aided relay network is proposed. Due to the limited buffer space of the relay node， it is necessary to study the allocation of buffer resources when the relay node needs to serve multiple users. To this end， this paper proposes a buffer resource allocation method for relay nodes. Firstly， according to the data arrival model and data processing model of the relay node， as well as the buffer overflow probability that the relay node can tolerate， the maximum buffer proportion contributed by the relay node is provided. Then， according to the user’s data arrival model and data service model， the expression between the user’s buffer overflow probability and the buffer portion is given. Then， an optimization problem is established to minimize the sum of user buffer overflow probability in the network， and the optimal buffer allocation method is presented. Finally， simulation results show that the proposed method can obtain a smaller sum of user buffer overflow probability than the uniform allocation method.